Download Math 107A Book - Sacramento State

Fundamental Mathematical Concepts
Math 107A
Professor T. D. Hamilton
January 17, 2007
2
Contents
1 Set Theory
What is a set? . . . .
Describing a Set . . .
2 vs.
. . . . . . . .
Set Operations . . . .
Union . . . . . .
Intersection . . .
Set Di¤erence . .
Complement . . .
Relationships . .
Properties . . . .
Venn Diagrams .
Equivalent Sets . . . .
Finite vs. In…nite
Chapter 1 Exercises .
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Sets
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2 Arithmetic of the Natural Numbers
Development of the Natural Numbers *** . . . . . . . .
Developing a New Number System (the fen system) ***
Activity –Fenland Economics I . . . . . . . . . . . . . .
Addition . . . . . . . . . . . . . . . . . . . . . . . . . . .
De…nition . . . . . . . . . . . . . . . . . . . . . . .
Algorithm *** . . . . . . . . . . . . . . . . . . . . .
Activity –Cuisinaire Rods . . . . . . . . . . . . . . . . .
Subtraction . . . . . . . . . . . . . . . . . . . . . . . . .
A First Model –Take-Away . . . . . . . . . . . . .
De…nition . . . . . . . . . . . . . . . . . . . . . . .
Algorithm *** . . . . . . . . . . . . . . . . . . . . .
Models for Subtraction . . . . . . . . . . . . . . . .
Activity –Place Value Game . . . . . . . . . . . . . . .
Activity –Base 10 Blocks . . . . . . . . . . . . . . . . .
Converting between Fen and Ten . . . . . . . . . . . . .
Multiplication . . . . . . . . . . . . . . . . . . . . . . . .
De…nition . . . . . . . . . . . . . . . . . . . . . . .
Array Model of Multiplication *** . . . . . . . . .
Algorithm *** . . . . . . . . . . . . . . . . . . . . .
Lattice Algorithm *** . . . . . . . . . . . . . . . .
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7
7
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9
13
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14
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15
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17
22
24
24
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27
27
28
28
29
29
30
31
31
31
32
32
32
34
35
36
38
38
38
38
38
3
CONTENTS
4
Activity –Fenland Economics II . . .
Division (without remainder) . . . . .
De…nition . . . . . . . . . . . . .
Models for Division . . . . . . . .
Division with Remainder . . . . . . . .
De…nition . . . . . . . . . . . . .
Algorithm *** . . . . . . . . . . .
Mental Arithmetic and Estimation ***
Chapter 2 Exercises . . . . . . . . . .
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38
39
39
41
42
42
43
43
43
3 Number Theory
Divisibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Primes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fundamental Theorem of Arithmetic . . . . . . . . . . . . . . .
Number of Divisors *** . . . . . . . . . . . . . . . . . . . . . .
Tests for Divisibility . . . . . . . . . . . . . . . . . . . . . . . .
Divisibility by 2, 5 and 10 *** . . . . . . . . . . . . . . . .
Divisibility by 3 and 9 *** . . . . . . . . . . . . . . . . . .
Divisibility by 11 *** . . . . . . . . . . . . . . . . . . . . .
Divisibility by 2r *** . . . . . . . . . . . . . . . . . . . . .
Divisibility Rules for Composites *** . . . . . . . . . . . .
Greatest Common Divisor (GCD) and Least Common Multiple
De…nitions . . . . . . . . . . . . . . . . . . . . . . . . . . .
Set Intersection Method . . . . . . . . . . . . . . . . . . .
Prime Power Representation Method *** . . . . . . . . .
Euclidean Algorithm Method *** . . . . . . . . . . . . . .
Activity –Interesting Numbers . . . . . . . . . . . . . . . . . .
Chapter 3 Exercises . . . . . . . . . . . . . . . . . . . . . . . .
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(LCM)
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51
51
52
56
57
57
57
57
57
57
57
57
57
58
58
58
59
60
4 The Integers
Opposites . . . . . . . . . . . .
Representations of the Integers
Activity –Colored Counters . .
Subtraction as Addition . . . .
Addition of Integers . . . . . .
Absolute Value . . . . . . . . .
De…nition . . . . . . . . .
Geometric Interpretation .
Inequalities . . . . . . . . . . .
De…nitions . . . . . . . . .
Geometric Interpretation .
Multiplication of Integers . . .
Division of Integers . . . . . . .
Chapter 4 Exercises . . . . . .
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65
65
66
67
69
70
72
72
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74
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75
76
77
78
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Preface
This course is designed to help to prepare you to teach mathematics in the elementary school
classroom. For this reason, we will be less concerned with how to do mathematics and more
concerned with why we do it the way we do. That is, why are the methods that we learned (and
teach) valid.
For example, I assume that you know how to compute 356 57: In this course we will discuss
why the technique you used to compute 356 57 is valid, that is, why you get the right answer.
We’ll also discuss other techniques and why they work. In the end you should be able to teach
your students how to multiply numbers and you should be able to present the material in such a
way that your students will understand why they are doing what they are doing. That way your
students will come away with a better understanding of mathematics. They will be able to explain
what they are doing without having to say, “I do it that way because my teacher said so.”
5
6
PREFACE
Chapter 1
Set Theory
Set theory is essentially just the study of collections of objects. It shows up throughout the grade
school mathematics curriculum. Consider how you were …rst taught to add 3 + 2: You probably did
something like this: if I have 3 apples and someone gives me 2 more apples, how many apples will
I have? In this example, you are taking a set with 3 objects and a set with 2 objects and …nding
the size of the union (see de…nition on page 13) of these two sets. We will come back to this idea
on page 29 when we discuss addition in more detail.
What is a set?
We will not have a rigorous de…nition of the word set. Instead, we will use the word set to describe
any collection of objects. The objects in the set are usually referred to as elements of the set. A
set does not have to be “mathematical.”
One important set is the set with no elements. This set is called the empty set and is often
denoted by the symbol ;: It is simply an empty collection. It is important to remember that the
empty set is not nothing; it is something, it is a set with no elements, i.e. a set with nothing in
it. Consider, for example, the set of all students at California State University Sacramento who
commute daily from New York City. This is a set, but there is certainly nobody in the set. Thus,
this set is an empty set. Another example would be to take the set of all numbers x such that
x 6= x: This set is also an empty set.
De…nition 1 We say that two sets are equal if they contain exactly the same elements.
The two empty sets mentioned in the previous paragraph are equal sets; they each contain no
elements, so, of course, they contain the same elements. Thus, the set of all students at California
State University Sacramento who commute daily from New York City is equal to the set of numbers
x such that x 6= x: In fact any empty set is equal to any other empty set. This is why we are able
to refer to the empty set rather than an empty set.
7
Set Theory
8
Now let’s consider an example of equal sets that aren’t empty.
Example 2 Let A be the set of all numbers x such that x2 = 1:
Let B be the set of all numbers x such that jxj = 1:
Notice that A contains the elements 1 and 1 and B contains the elements 1 and
A = B:
1: Therefore,
Describing a Set
There are many ways of describing a set.
So far we have been describing sets in words. That is, we have simply described the conditions
necessary for an object to be an element of the set.
Another option is to describe a set in roster notation. A roster is simply a list and, in roster
notation, you simply list the elements of the set inside set brackets, f g :
Example 3 Recall the set A from the previous example. In words we described A as the set of all
numbers x such that x2 = 1: In that example we said that A contains the elements 1 and 1: So,
if we want to describe A in roster notation, we would write A = f1; 1g : Note that we could also
put A in roster notation by writing A = f 1; 1g :
Notice that if the set is a (small) …nite set and you know the elements of the set, then roster
notation is a very simple way of describing the set.
Since the empty set is a set with no elements, we can write the empty set in roster notation as
f g : That is, ; = f g : In fact, this is the de…nition of the empty set.
De…nition 4 The set f g ; the set without any elements, is called the empty set and can be
denoted by the symbol ;:
;=f g
Note that ; is not equal to f;g since f;g is a set (given in roster notation) with one element,
;: One analogy that helps in understanding the di¤erence between these two sets is to think of a
set as a bucket with the objects of the set inside the bucket. So, f1; 1g would be a bucket with
two objects in it, the numbers 1 and 1: Using this analogy, ; would be an empty bucket while
f;g would be a bucket containing an empty bucket (one bucket inside of another).
When you describe a set in roster notation, it is easy to determine how many elements a set
has and what those elements are. However, to make these decisions easy, you must describe the set
without redundancy. For example, the set f1; 1; 1g is also equal to A: That is, A = f1; 1; 1g :
Why is this true? Remember that, by de…nition, two sets are equal if they contain exactly the same
elements. In this case, A contains two elements, the numbers 1 and 1 and the set f1; 1; 1g also
contains two elements, the numbers 1 and 1: So, A = f1; 1; 1g :
If we write A as f1; 1; 1g ; we say that we have redundancies. When we write A = f1; 1g ;
we say that we have put A in roster notation without redundancy.
p
Example 5 Let B = 3; 6; 15
; 36; 18 15; 3 :
5
p
Since 15
36 = 6 and 18 15 = 3; we have B = f3; 6; 3; 6; 3; 3g :
5 = 3 and
If we want to write B is roster notation without redundancy, we write B = f3; 6g :
So, even though it originally appeared as if B had 6 elements, we found that B actually only
contains 2 elements.
Set Theory
9
Example 6 Let C = f;; f g ; f inishg
A …nal option is to describe a set in set-builder notation. The basic format for set-builder
notation is as follows,
fx : conditiong
and is read “the set of all x such that condition holds.”The condition will change with the set.
Think of it as the condition which determines residency in the set, that is, it is the condition that
an object must satisfy to be in the set.
Example 7 Let’s write the set A, from the previous examples, in set-builder notation. Since A is
the set of all numbers x such that x2 = 1; the condition which determines residency in A is x2 = 1
So, A = x : x2 = 1 : To read this you would say “A equals the set of all x such that x2 = 1:”
Set-builder notation for a set is not unique. Notice that the set in the example above could also
be written as fx : jxj = 1g (read “the set of all x such that the absolute value of x is equal to 1”)
or fx : x = 1 or x = 1g (read “the set of all x such that x is equal to 1 or x is equal to 1:”)
2 vs.
We are now ready to introduce some notation.
We will start with the symbol, 2 : This symbol is used to denote “is an element of.”
Also, we use the symbol 62 to denote “is not an element of.”
Example 8 Let A = f1; 1g : Then A has two elements, 1 and
Also, since 2 is not an element of A; we can write 2 62 A:
The other notation,
1: So, we have 1 2 A and
1 2 A:
, is used to denote “is a subset of.” For this we need a de…nition.
De…nition 9 Let X and Y be sets. We say that X is a subset of Y; written X
element of X is also an element of Y:
If X is not a subset of Y; we write X 6 Y:
Y; if every
Example 10 Let A = f1; 1g and let B = f 2; 1; 0; 1; 2g : Then A has two elements, 1 and 1:
B has …ve elements, 2; 1; 0; 1 and 2: So, every element of A is an element of B: So, A B:
Notice however that not every element of B is an element of A since 2 2 B; but 2 62 A: Thus,
B 6 A:
Now, for a more complex example. In the next example, we will have a set whose elements
are sets. This situation can seem confusing as …rst, but it is really no di¤erent from the previous
examples.
Example 11 Let C = f;; 1; f1; 2g ; f2g ; 3; 4g : Before doing anything else, let’s determine the elements of C: Since C is given in roster notation, we know that the elements of C are inside the
outer set brackets and seperated by commas. I like to list the elements in a vertical list so that I
can remove the seperating commas and see more clearly what the elements of C are.
Set Theory
10
The elements of C are
;
1
f1; 2g
f2g
3
4
There is no redundancy in this list, so C contains 6 elements. In words:
the empty set
the number 1
the set containing the numbers 1 and 2
the set containing the number 2
the number 3
the number 4.
True or False: 1 2 C
This is true since the number 1 is on the list of elements of C:
True or False: 1 C
This is false since the number 1 is not a set, so it cannot be a subset of C:
True or False: 2 2 C
This is false since the number 2 is not on the list of element of C:
True or False: f2g 2 C
This is true since f2g ; the set containing the number 2, is on the list of elements of C:
True or False: f2g C
This is false since the set f2g contains the element 2; but C does not contain the element
2: That is 2 2 f2g ; but 2 62 C: In order to have f2g C; we need to have every element of f2g be
an element of C:
True or False: f1; 2g C
This is false since 2 2 f1; 2g but 2 62 C: In order for this to be true we would need every
element of f1; 2g (that is: the number 1 and the number 2) to be an element of C: While the number
1 is an element of C; the number 2 is not. So we do not have every element of f1; 2g in C: So,
f1; 2g 6 C:
The concept of subset shows up in the way that we learn numbers as a child.
We start with the simplest numbers, the counting numbers. The name for these numbers is the
natural numbers. The symbol for the set of natural numbers is N: So, in roster notation,
N = f1; 2; 3; 4; : : :g :
Then we build up, by including 0, to the whole numbers. The symbol for the set of whole
numbers is W: So, in roster notation,
W = f0; 1; 2; 3; 4; : : :g :
Notice that every element of the natural numbers, N; is an element of the whole numbers, W:
So, N W:
Set Theory
11
Next, we learn about the negative numbers and build up to the integers. The symbol for the
set of integers is Z: So, in roster notation,
Z = f: : : ; 2; 1; 0; 1; 2; : : :g :
Notice that every element of W is an element of Z; so we have W Z: (we also have N Z)
Next we learn about fractions, the rational numbers. The symbol for the set of rational
numbers is Q: It would be very di¢ cult to give Q in roster notation, so we usually describe this set
in set-builder notation,
o
na
: a 2 Z and b 2 Z and b 6= 0 :
Q=
b
So, Q contains all numbers that can be written as ab where a and b are integers and b is not
equal to 0.
Notice that if n is an element of Z; then n = n1 , so n is an element of Q: Therefore, every
element of Z is an element of Q: So, Z Q:
We next move up to the real numbers. The symbol for the set of real numbers is R: The set
of real
p numbers includes all of the numbers that we’ve mentioned so far. It also contains numbers
like 2 and : The set of real numbers contains all numbers on the number line. Since all rational
numbers are real numbers, we have Q R:
Finally, we learn about the imaginary
p numbers. For these we usually start with the number i
which is de…ned by i2 = 1, that is i =
1: With this we can build the set of complex numbers,
denoted C: The set of complex numbers includes all numbers of the form a + bi where a and b are
real numbers. In set-builder notation this becomes
C = fa + bi : a 2 R and b 2 Rg :
Suppose r is a real number, then r = r + 0i which is an element of C: Thus, every element of
R is an element of C: So, R C:
Putting all of this together, we can describe the progressing of numbers learned in grade school
in one line:
N W Z Q R C:
Arithmetic Sequences
One type of set that we will have a special interest in is a set whose elements form an arithmetic
sequence.
De…nition 12 A sequence of numbers is called an arithmetic sequence if each term of the
sequence is found by adding a …xed value (called the common di¤ erence) to the previous term.
Example 13 Each of the following sequences is an arithmetic sequence.
3,7,11,15,19,. . . (common di¤ erence = 4)
8,13,18,23,. . . (common di¤ erence = 5)
7,5,3,1,-1,-3,. . . (common di¤ erence =
2)
Example 14 Each of the following sequences is not an arithmetic sequence.
3,7,12,17,22,. . . (add 4 to term 1 to get to term 2, but add 5 to get from term 2 to term 3)
Set Theory
12
2,4,8,16,. . . (add 2 to term 1 to get to term 2, but add 4 to get from term 2 to term 3)
We want to be able to take a set, given in roster notation, whose elements form an arithmetic
sequence and give the set-builder notation for that set.
Example 15 Let X = f3; 7; 11; 15; 19; : : :g : So, X is a set, given in roster notation, whose elements
form an arithmetic sequence.
If we want to …nd a useful set-builder notation form of this set, we will need to …nd a formula
relating the elements of X:
Notice that the …rst element of this set is 3:
After that, we add 4 to 3 to get 7:
Then, we add 4 to 7 to get 11. Adding 4 to 7 is the same as adding 4 twice to 3.
Notice that if we add 4 three times to 3 we get 15, the fourth element of the set.
So, each element of this set is found by starting at the number 3 and adding some number of
4’s. That is, they all are of the form 3 + 4n:
The …rst element, 3; is 3 + 4 0: (n = 0)
The second element, 7; is 3 + 4 1: (n = 1)
The third element, 11; is 3 + 4 2: (n = 2)
Etc.
So, every element of this set is of the form 3 + 4n where n is a whole number.
Thus, X = fx : x = 3 + 4n where n 2 Wg :
(Read this: X is equal to the set of all x such that x is equal to 3 plus 4n where n is a whole
number.)
In the previous example, we were successful in writing the set X in set-builder notation. There
is one problem though with our …nal answer. Notice that the …rst element of the sequence, 3;
corresponds with n = 0: It would be nice if the …rst element corresponded to n = 1: Suppose that
you were asked for the 100th term of this sequence. With what we have done so far you would
have to realize that the 100th term of this sequence corresponds to n = 99: So, the 100th term is
x = 3 + 4 (99) = 399: Let’s see if we can …x this problem. That is, we want to write X is set-builder
notation so that the …rst term corresponds to n = 1; the second term to n = 2; etc.
Example 16 Continuing the previous example. We know that all terms of this sequence are found
by adding some multiple of 4 to the number 3. Let’s look at the …rst few terms.
term 1
= 3 = 3 + 4 0 = 3 + 4 (1
1) = 3 + 4 (n
1) where n = 1
term 2
= 7 = 3 + 4 1 = 3 + 4 (2
1) = 3 + 4 (n
1) where n = 2
term 3
= 11 = 3 + 4 2 = 3 + 4 (3
1) = 3 + 4 (n
1) where n = 3
It appears that this pattern will continue. So, we have that every term of the sequence is of the
form 3+4 (n 1) where n is a natural number. Also, this time the value of n is equal to the position
of the term in the sequence. That is, n = 1 corresponds to the …rst term, n = 100 corresponds to
the 100th term, etc.
So, X = fx : x = 3 + 4 (n 1) where n 2 Ng : We should simplify our formula. Notice that
3 + 4 (n 1) = 3 + 4n 4 = 4n 1: So,
X = fx : x = 4n
1 where n 2 Ng :
Now, if we want to determine the 100th term of this sequence, we simply use n = 100 in the
formula above to get x = 4 (100) 1 = 399:
Set Theory
13
Set Operations
In this section, we will learn how to create new sets from given sets.
Union
Our …rst set operation is the union of sets.
De…nition 17 Let A and B be sets. The union of A and B; denoted A [ B; is the set containing
all elements which are in at least one of the two original sets. In set builder notation we have
A [ B = fx : x 2 A and/or x 2 Bg :1
(1.1)
Example 18 Let A = f 1; 1g and let B = f0; 1; 2; 3g : Then A [ B is the set of all elements that
are in at least one of the two original sets. So, A [ B = f 1; 0; 1; 2; 3g : Notice that the number 1
is contained in both of the original sets. We do not list it twice in A [ B since we are giving A [ B
in roster notation without redundancy.
Sometimes we can determine information about sets simply by being told the union of the sets
as in the example below.
Example 19 Suppose that A; B and C are sets and that A [ B = f1; 2; 3g and A [ C = f1; 2; 3; 5g
and B [ C = f1; 2; 3; 5g :
Since A [ B does not contain the number 5; we know that the number 5 is not an element of A
and is not an element of B:
However, 5 is an element of A [ C; so 5 must be an element of A and/or of C: Since 5 is not
an element of A; it must be that 5 is an element of C:
In this example, we cannot determine much more than this. In fact, here are two possibilities
for A; B and C:
1. A = f1; 2g ; B = f2; 3g ; and C = f1; 3; 5g :
2. A = f1g ; B = f2; 3g ; C = f1; 2; 3; 5g :
Question: Can you …nd another choice of A, B and C that satisfy the conditions of this
example, i.e. A [ B = f1; 2; 3g and A [ C = f1; 2; 3; 5g and B [ C = f1; 2; 3; 5g?
Intersection
De…nition 20 Let A and B be sets. The intersection of A and B; denoted A \ B; is the set
containing all elements which are in both of the original sets. That is, A\B contains those elements
common to both A and B: In set builder notation we have
A \ B = fx : x 2 A and x 2 Bg :2
(1.2)
Example 21 Let A = f 1; 1g and let B = f0; 1; 2; 3g : Then A \ B is the set of all elements that
are common to the two sets. The only element common to both is the number 1. So, A \ B = f1g :
p
Example 22 Let C =
1; 12 ; 2 and D = N = f1; 2; 3; 4; : : :g : Then C \ D is the set of all
elements that are common to the sets. Since there are no elements common to these two sets, we
have C \ D = ;:
1
2
This is read “A union B is equal to the set of all x such that x is an element of A and/or x is an element of B:”
This is read “A intersect B is equal to the set of all x such that x is an element of A and x is an element of B:”
Set Theory
14
Set Di¤erence
De…nition 23 Let A and B be sets. The set di¤ erence of A and B; denoted A B; is the set
containing all elements which are in A but are not in B: In set builder notation we have
A
B = fx : x 2 A and x 62 Bg :
(1.3)
Example 24 Let A = f 1; 1g and B = f0; 1; 2; 3g :
Then A B is the set of all elements in A that are not in B: The elements of A are 1 and 1:
The only one of these that is in B is 1: So, A B = f 1g :
Similarly, B A is the set of all elements in B that are not in A: The elements of B are 0; 1;
2 and 3: The only one of these that is in A is 1: So, B A = f0; 2; 3g :
p
Example 25 Let C =
1; 21 ; 2 and D = N = f1; 2; 3 : : :g :
p
Then C D =
1; 12 ; 2 since none of the elements of C are in D:
Similarly, D C = f1; 2; 3; : : :g :
Example 26 Let X = f1; 2; 3; 4; 5g and let Y = N = f1; 2; 3 : : :g :
Then X Y contains the elements of X that are not in Y: However, every element of X is an
element of Y; so there are no elements of X that are not in Y: Thus, X Y = ;:
On the other hand, Y X contains the elements of Y that are not in X: So, Y X = f6; 7; 8; : : :g :
Complement
Our …nal set operation is the complement of a set. The complement of a set can be thought of as
the set of things needed to “complete” the set. In order to understand what this means, we have
to understand what we mean by complete. For this, we introduce the concept of a universe. The
universe is a universal set which all other sets, within a particular discussion, are a subset of.
Example 27 Suppose we are discussing the sets A = f1; 1g ; B = f1; 3; 5g and C = f0; 5g : We
can not tell from this information what the universe is. However, each of these sets must be a
subset of the universe. So, the smallest possible universe is f 1; 0; 1; 3; 5g :
On the other hand, it is possible that the universe is Z: Or, the universe could be R: Or, the
universe could be fx : x 2 Z and
5 x 5g = f 5; 4; 3; 2; 1; 0; 1; 2; 3; 4; 5g :
As you can see from the previous example, the universe cannot be determined uniquely from
the given sets.
De…nition 28 Let A be a set. The complement of A; denoted A; is the set containing all
elements of the universe which are not in A: In set builder notation we have
A = fx : x 2 U and x 62 Ag :
(1.4)
Where U denotes the universe.
Since it is generally assumed that all elements are in the universe (that’s why it’s called the
universe), this is sometimes written as
A = fx : x 62 Ag :
Set Theory
15
Notice that A is simply the set of objects that you need to combine with the set of objects in
A to get the universe. That is
A [ A = U:
Also, by de…nition of A; no element can be in both A and A: So,
A \ A = ;:
Finally, notice that if we change the universe, we change the complement of a set.
Example 29 Let A = f1; 2; 3g : We cannot compute A until we are told what the universe is.
1. Suppose U = f1; 2; 3; 4; 5g : Then A contains those elements of f1; 2; 3; 4; 5g that are not in
A: So, A = f4; 5g :
2. Suppose U = N: Then A = f4; 5; 6; : : :g :
3. Suppose U = W: Then A = f0; 4; 5; 6; : : :g :
4. Suppose U = Z: Then A = f: : : ; 2; 1; 0; 4; 5; 6; : : :g :
Relationships
You may have realized by now that there is a strong relationship between set-di¤erence and complement. In this section we will make this relationship clear.
In the previous section, we de…ned A = fx : x 2 U and x 62 Ag :
According to equation 1.3 in the section on set di¤erence, the de…nition of U A is U A =
fx : x 2 U and x 62 Ag :
So,
A = U A:
Now, consider A B: Recall that A B is the set of objects that are in A but are not in B:
Also, consider A \ B: This is the set of objects in both A and B: However, if an object is in B;
then it is not in B (see the de…nition of B): So, if an object is in both A and B; then it is both in
A and not in B: So, A \ B contains those objects that are in A and not in B: This is the same as
the description of A B: So,
A B = A \ B:
One advantage of this last relationship is that, to compute A B; you do not need to know the
universe. So, you can compute A \ B even without knowing the universe by using the fact that
A \ B = A B:
Properties
In this section we will be studying some of the properties of these set operations. Throughout
this course we will be studying properties of operations (e.g. union of sets, addition of integers,
multiplication of real numbers, etc.), so we will lay out the general versions of these properties
here. Each property is a property of an operation (or two) on a collection of objects. The general
statements below are written in terms of a generic operation and set of objects.
Commutative Property of Operation of Objects : For any two objects a and b;
a ~ b = b ~ a:
Set Theory
16
Associative Property of Operation of Objects : For any three objects a; b and c;
(a ~ b) ~ c = a ~ (b ~ c) :
Distributive Property of Operation 1 over Operation 2 of Objects : For any three objects
a; b and c;
a ~1 (b ~2 c) = (a ~1 b) ~2 (a ~1 c) :
Closure of Operation of Objects : For any two objects a and b;
a ~ b is an object.
Using the generic statements above as models, we can state some speci…c properties. Note,
simply stating a property does not make it true. In each case we will have to ask ourselves whether
or not the property is a true property. If a property is true, we will have to make a general argument
stating why it is true. This is because each of the properties that we are studying are statements
about all objects. So, the property is not true just because the equation “works”for a few objects,
it must work for all of them. On the other hand, suppose we want to argue that one of these
properties is false. In that case, we have to show that the equation is not always true. So, we
simply need to …nd a counterexample; that is, a speci…c example of objects for which the prescribed
equation is not true. If we can do this, then the equation cannot be true for all objects, so we have
shown that the property is false.
Example 30 State the commutative property of unions of sets. In this case the operation is union,
[; and the objects are sets. So,
The commutative property of unions of sets: For any two sets A and B; A[B = B[A:
Is this a true property? Suppose A and B are sets. Then A [ B is the set of objects that are in
A and/or in B: Clearly, this is the same as the set of objects that are in B and/or in A, that is
B [ A: Thus, A [ B = B [ A: So, this is a true property.
Example 31 State the commutative property of subtraction of integers. (This is not a set theory
example, but we will be studying this type of property later.) In this case the operation is subtraction,
; and the objects are integers. So,
The commutative property of subtraction of integers: For any two integers a and
b; a b = b a:
Is this a true property? No. Let a = 3 and b = 2: Then a and b are integers, but a b = 3 2 = 1
and b a = 2 3 = 1; so a b 6= b a: Therefore, this is a false property.
Example 32 State the associative property of intersection of sets. In this case the operation is
intersection, \, and the objects are sets. So,
The associative property of integersection of sets: For any three sets A; B and C;
(A \ B) \ C = A \ (B \ C) :
Is this a true property? Yes, for any sets A, B and C; both sides of the equation above are sets
containing all elements common to the three original sets. This is a true property.
Example 33 State the distributive property of union over set di¤ erence of sets. In this case operation 1 is union, [; operation 2 is set di¤ erence, ; and the objects are sets.
The distributive property of union over set di¤ erence of sets: For any three sets
A; B and C; A [ (B C) = (A [ B) (A [ C) :
Set Theory
17
Is this a true property? No. Let A = f1; 2; 3g ; B = f2; 3; 4g and C = f3; 4; 5g : Then
A [ (B
C) = f1; 2; 3g [ (f2; 3; 4g
= f1; 2; 3g [ f2g
f3; 4; 5g)
= f1; 2; 3g
and
(A [ B)
(A [ C) = (f1; 2; 3g [ f2; 3; 4g)
= f1; 2; 3; 4g
= ;:
So, A [ (B
C) 6= (A [ B)
(f1; 2; 3g [ f3; 4; 5g)
f1; 2; 3; 4; 5g
(A [ C) : So, this is a false property.
Example 34 State the closure property of addition of natural numbers. In this case the operation
is addition, +; and the objects are natural numbers. So,
Closure of addition of natural numbers: For any two natural numbers a and b; a + b
is a natural number.
Is this a true property? Yes. The sum of any two natural numbers is a natural number. This
is a true property.
Example 35 State the closure property of subtraction of natural numbers. In this case the operation is subtraction, ; and the objects are natural numbers. So,
Closure of subtraction of natural numbers: For any two natural numbers a and b;
a b is a natural number.
Is this a true property? No. Let a = 3 and b = 5; then a b = 3 5 = 2 is not a natural
number. So, this is a false property.
Note that the example above does not show us that closure of subtraction is false. It shows us
that closure of subtraction of natural numbers is false. Consider the following example:
Example 36 State the closure property of subtraction of integers. In this case the operation is
subtraction, ; and the objects are integers. So,
Closure of subtraction of integers: For any two integers a and b; a b is an integer.
Is this a true property? Yes. If you choose any integer and subtract any other integer from it,
the result will be an integer. This is a true property.
Venn Diagrams
Venn diagrams give us a visual representation of generic sets and set properties. We will use venn
diagrams to help us prove (or disprove) some properties of set operations like those studied in the
previous section. When discussing two sets, say A and B, we start with the basic venn diagram
below
Set Theory
18
Where the rectangle represents the universe and the two circles represent the two sets. Notice
that there are four regions in this picture. Each region corresponds to a possibility for where an
object in the universe lives with respect to A and B:
1. An object may be in neither of the two sets A and B: In this case, the object would be in the
region shaded below.
2. An object may be in set A; but not in set B: In this case, the object would be in the region
shaded below.
3. An object may be in set B; but not in set A: In this case, the object would be in the region
shaded below.
4. Finally, an object may be in both set A and set B: In this case, the object would be in the
region shaded below.
When discussing three sets, say A; B and C; we simply need to include a third circle to represent
the third set. How many regions should we end up with? To determine the answer to this question,
let’s consider all of the possibilities for where an object in the universe can live with respect to the
sets A; B and C:
1. The object may be in none of the sets A; B and C:
2. The object may be in set A; but not in set B and not in set C:
3. The object may be in set B; but not in set A and not in set C:
Set Theory
19
4. The object may be in set C; but not in set A and not in set B:
5. The object may be in sets A and B; but not in set C:
6. The object may be in sets A and C; but not in set B:
7. The object may be in sets B and C; but not in set A:
8. The object may be in all three sets, A; B and C:
So, there are eight possibilities. So, the venn diagram should have eight regions. The standard
way of representing this is as follows.
Exercise 37 Identify which region in the venn diagram above corresponds to each of the eight
possibilities listed above.
It is very di¢ cult to draw an appropriate venn diagram when four or more sets are being
discussed. With four sets, there would need to be 16 regions. Can you determine, as we did above
for two sets and for three sets, what the 16 possibilities are? For …ve sets, there would have to be
32 regions! So, we usually use venn diagrams to help us when studying situations with two or three
sets.
Our primary use of venn diagrams will be to prove or disprove properties of set operations.
Example 38 Earlier (see page 15) we argued that, for any sets A and B; we have A B =
A \ B: Now, let’s prove it using venn diagrams. If this statement is true, then the venn diagram
representing A B should be identical to the venn diagram for A \ B. Since there are only two sets
involved, we will be using the basic two set venn diagram.
In the venn diagram for A B; we need to shade all regions containing elements in A B:
Since A B contains all elements in A but not in B; the diagram is
A
B
Set Theory
20
Now, we need to …nd the venn diagram for A \ B: This will take two steps since there are two
operations here, complement and intersection.
We start with B: The diagram for B should have all regions shaded containing elements of B;
that is all regions containing elements not in B:
B
Now, A \ B contains all elements common to both A and B: So, we look at the diagram for B
and determine which regions shaded there are common to A to get
A\B
Notice that the venn diagram for A B is the same as the venn diagram for A \ B: So, for any
two sets A and B; we have A B = A \ B:
Now, let’s use venn diagrams to determine whether a property is true or false.
Example 39 Is the associative property of set di¤ erence of sets a true property?
To answer this let’s …rst state the property3 : For any three sets A; B and C; (A
A (B C) :
So, we want to compare the venn diagrams for (A B) C and A (B C) :
Let’s start with A B: We want objects/regions in A but not in B:
A
3
B
Refer to page 16 if you need a reminder on associative property statements.
B)
C =
Set Theory
21
So, the venn diagram for (A
B)
C will have regions shaded that are in A
(A
Now, we need to work on A
We start with B C:
(B
B)
(B
C
C) :
B
So, A
B; but not in C:
C
C) will be those regions in A not shaded in the diagram of B
A
(B
C above.
C)
Since the venn diagrams for (A B) C and A (B C) are not the same, we have that the
associative property of set di¤ erence of sets is a false property.
When we …rst discussed these properties, we decided that, to show that a property was false,
we needed to give a counterexample. We can use the venn diagrams determined above to help us
come up with a counterexample for the associative property of set di¤erence of sets.
Example 40 Use the information gathered in the previous example to help determine a counterexample to the associative property of set di¤ erence of sets, that is, to determine sets A; B and C
such that A (B C) 6= (A B) C:
Set Theory
22
If we look at the venn diagrams for A (B C) and (A B) C we see that there are two
regions shaded in A (B C) that are not shaded in (A B) C: So, for our counterexample, we
need to make sure that there are elements in at least one of these regions. Notice that both of these
regions include elements that are in both A and C: So, let’s choose sets so that A and C have some
common elements.
Let’s start with A = f1; 2; 3g : (I chose this randomly – something simple.)
We didn’t determine any requirements on B above, so let’s stick with something simple here
too. Let B = f3; 4g :
We need A and C to have common elements, so let’s choose C = f1; 4g :
Now, let’s see if we have really determined a counterexample.
A
(B
C) = f1; 2; 3g
(f3; 4g
= f1; 2; 3g
f3g
= f1; 2g :
f1; 4g)
And
(A
B)
C = (f1; 2; 3g
= f1; 2g
= f2g :
So, A
(B
C) 6= (A
B)
f3; 4g)
f1; 4g
f1; 4g
C:
Equivalent Sets
Recall that two sets are equal if they contain exactly the same elements. So, f1; 2; 3g is equal to
f2; 3; 1g since both of these sets contain the elements 1; 2 and 3: While the set f1; 2; 3g is NOT
equal to the set fa; b; cg ; these two sets do have something in common. They each have three
elements. In fact, these sets can be used to help us understand the concept of “three.”
Ask yourself how you would respond if a child asked you, “what does 3 mean?” You would
probably show him many collections of three objects. You might hold up three …ngers, place three
pennies on his desk, draw a picture of three apples, etc. That is, you would show the child many
sets with three elements each. These sets are called equivalent sets.
You would also probably show the child how these collections are related. You might take your
three …ngers and place one …nger on each of the three pennies. In this way you are demonstrating
the “threeness” shared by these two collections. When you do this you are demonstrating a oneto-one correspondence between the two sets.
De…nition 41 Two sets, A and B; are said to be equivalent , written A
one-to-one correspondence between the elements of the two sets.
Example 42 Show that f1; 2; 3g is equivalent to fa; b; cg :
We need to demonstrate a one-to-one correspondence between these two sets:
1
2
3
! a
! b
! c
B; if there is a
Set Theory
23
Note that there will generally be more than one one-to-one correspondence between two equivalent sets. For example, in the example above, we could have chosen
1
2
3
! c
! a
! b
instead. We simply have to make sure that we use every element of each set and that no element
is corresponded to more than one other element.
It is fairly simple, as shown above, to show that two …nite sets are equivalent. We simply
describe the correspondence completely. What if the sets were in…nite? We could never list all of
the corresponding elements.
Example 43 Let A = f2; 4; 6; : : :g and let B = f5; 8; 11; : : :g : Show A
An obvious correspondence would be
2
4
6
B:
! 5
! 8
! 11
..
.
However, this is an incomplete description. For example, 20 2 A; so 20 must correspond to
some element of B: Which one? That is 20 !???
We need to come up with a formula to describe this correspondence completely and generically.
Each of these sets contains elements forming an arithmetic sequence (see page 11). Using the
techniques we developed for writing these types of sets in set-builder notation we get
A = fx : x = 2 + 2n where n 2 Wg ; and
B = fx : x = 5 + 3n where n 2 Wg :
It seems that the generic correspondence should be
2 + 2n
! 5 + 3n (where n 2 W):
We can check this by plugging in values for n to see if we get the correspondence we started this
example with.
When n = 0; we get 2 ! 5: When n = 1; we get 4 ! 8. Etc.
Example 44 Let A = fx : x = 3n + 1 where n 2 Ng and let B = fx : x = 5n + 3 where n 2 Wg :
Show A B:
Before we do anything else, let’s make sure we understand these sets. Let’s write each in roster
notation.
The elements of A are of the form 3n + 1 where n is a natural number. When n = 1; 3n + 1 = 4:
When n = 2; 3n + 1 = 7: When n = 3; 3n + 1 = 10: Etc. So, A = f4; 7; 10; : : :g :
Similarly, B = f3; 8; 13; : : :g :
As before, we expect our correspondence to be
4
7
10
! 3
! 8
! 13
..
.
Set Theory
24
Based on the initial descriptions of A and B; we might want to use the generic correspondence
3n + 1
! 5n + 3:
Notice that this does not give us the speci…c correspondence we expected. For example, when
n = 1 this gives us 4 ! 8; but we expected 4 ! 3:
Why didn’t this work this time. In our previous example, the value of n came from the same set
(W) : However, this time n 2 N in A; but n 2 W in B: So, to …x this we simply need to write both
sets A and B with the same indexing set (i.e. both sets using n 2 N or both sets using n 2 W):
Set A is a set whose elements form an arithmetic sequence. Using the technique we learned for
writing such a set in set-builder notation, we get
A = fx : x = 4 + 3n where n 2 Wg :
Now we have A = fx : x = 4 + 3n where n 2 Wg and B = fx : x = 5n + 3 where n 2 Wg : So,
the generic correspondence is
4 + 3n
! 5n + 3 where n 2 W:
Finite vs. In…nite Sets
We used the terms …nite set and in…nite set in the previous section. Now, it’s time to give a careful
de…nition of these terms.
De…nition 45 A set A is said to be …nite if
1. A
f1; 2; : : : ; ng for some natural number n; or
2. A = ;:
Note that A f1; 2; : : : ; ng basically means that the elements of A can be counted o¤. It says
that the elements of A can be corresponded to the numbers 1, 2, 3, . . . , n: So, there is a 1st element,
2nd element, . . . , nth element of A:
Example 46 Let A = fa; b; cg : Then A is a …nite set since, as we showed in example 42 on page
22, A f1; 2; 3g :
De…nition 47 If a set A is not a …nite set, then A is an in…nite set.
Chapter 1 Exercises
1. List each set in roster notation.
(a) A = fx : x = 4n + 5 where n is a natural numberg
23
(b) B = x : x is a whole number and 4:7 x <
2
2. List each set in set builder notation.
Set Theory
25
(a) D = f3; 7; 11; 15; : : :g
(b) E = f1; 7; 13; 19; 25; : : :g
3. Let S = ffag ; b; fbg ; fa; bgg : Label each of the following as true or false.
(a) a 2 S
(b) b 2 S
(c) a
S
(d) fag 2 S
(e) fag
S
(g) fbg
S
(f) fbg 2 S
(h) fa; bg 2 S
(i) fa; bg
S
(j) ffagg
S
(k) fa; fbgg
S
(l) ffag ; bg
S
4. A = f ; 4; ~; f~g ; f ; 4gg Determine whether each of the following sets is a subset of A;
an element of A; both a subset and an element of A or neither a subset nor an element of A:
(a) f~g
(b) f ; 4g
(c) f ; f~gg
(d) f ; 4; ~g
5. U = f;; a; fag ; fa; bg ; b; c; d; e; ff gg ; 1; 2; f2gg
A = f;; 1; a; c; eg
B = f;; ffgg ; a; b; fa; bg ; eg
C = f1; 2; f2g ; f;g ; fa; bgg
Find each of the following. List each set without redundancy.
(a) A [ B
(b) A \ C
(c) B \ C
(d) B [ C
(e) C \ A [ B
6. Let A =
1
2;
p
2; 10 ; B =
1 3
2; 2
; C = ;; and D = f5; 10; 15g :
(a) Determine the smallest possible universe for this collection of sets.
(b) Determine A (using the universe you found in part a)
Set Theory
26
(c) Determine C:(using the universe you found in part a)
7. Let A = fx : x is a natural numberg and let B = fx :
C in roster notation.
x < 10g : Let C = A \ B: Specify
8. State each of the following set properties.
(a) Commutative property for intersection of sets.
(b) Associative property for unions of sets.
(c) Distributive property for intersection over union of sets.
9. Complete exercise 37 (regarding the three set venn diagram) on page 19
10. Draw a Venn diagram to represent each of the following. For your …nal answer draw a diagram
with only the appropriate region shaded.
(a) A \ B
(b) A [ B
11. Use Venn Diagrams in each part to determine whether or not the two sets are equal.
(a) (A [ B); A [ B
(b) (A [ B); A \ B
12. U = fa; e; f; i; j; f1g ; 2; f2g ; 3; f g ; f;gg
A = fa; ff gg ; f2g ; ig ; B = fa; 2; ;; 3; f gg ; C = fe; f; f2g ; f1g ; ;; 2; 5
2g
In each part determine whether or not the two sets are equivalent. Give an explicit 1-1
correspondence for each equivalent pair or justify your answer if the sets are not equivalent.
(a)
(b)
(c)
(d)
A; B
C, A
C B; A
A [ B; C
n
o
23
13. A = f;g ; 4; fxg ; f g ; 36 ; B = f2; x; ;; p; f gg ; and C = 5; 6; fyg ; fxg ; ;; x; p
4
Determine whether or not each of the following pairs is equivalent and justify each answer.
Give a 1-1 correspondence for each equivalent pair.
(a)
(b)
(c)
(d)
A; B
A [ B; C
C; A [ B
A \ C; B
B
C
14. Let A = f3; 9; 15; 21; 27; : : :g : Let B = fx : x = 4n
3 where n is a natural numberg :
(a) Specify A using set builder notation.
(b) Specify B using roster notation.
(c) Are sets A and B equivalent? If so, …nd a 1-1 correspondence between A and B:
Chapter 2
Arithmetic of the Natural Numbers
Development of the Natural Numbers ***
oral beginnings
historical breakthrough of place value systems
– need for a place holder
later development of 0 as “nothing”
development of N and W as cardinal numbers of a set
27
Arithmetic of the Natural Numbers
28
Developing a New Number System (the fen system) ***
oral beginnings
how many words needed?
place value
even/odd numbers
Activity –Fenland Economics I
In Fenland, they have four denominations of money.
The lowest denomination is blue.
Five blues is worth a yellow.
Five yellows is worth a green.
Five greens is worth a pink.
Bank tellers in Fenland are trained to make only one exchange at a time. That is, a teller will
make any of the four exchanges listed above, but will not combine them. For example, you can take
a green to the bank and the teller will give you …ve yellows in exchange for it. The teller WILL
NOT give you four yellows and …ve blues in exchange for a green. If you want to make such an
exchange you will have to do it one exchange at a time. That is, you can exchange your green for
…ve yellows. You can, then, exchange one of your yellows for …ve blues. You should keep track of
all exchanges you make as you work through the following activity.
1. You start with 3 greens, 2 yellows and 4 blues. Draw a …gure representing this.
2. On payday you receive 2 green, 2 yellow and 2 blue. Using the smallest number of bills
possible, describe how much you now have. Describe any exchanges you make along the way
to determining the answer.
3. You receive a bill for 3 green, 1 yellow and 3 blue. After paying the bill, how much are you
left with. Use the smallest number of bills possible to describe your answer. Describe any
exchanges you make along the way to determining the answer.
Arithmetic of the Natural Numbers
29
This Fenland economics activity gives us a nice model for fen numbers. Notice that in the fen
number system
The lowest place value is the ones.
Five ones is a fen.
Five fens is a fefen.
Five fefens is a fefefen.
So, for example, 2 pink, 3 green, 1 blue models the number two fefefen, three fefen, one, that is
2301f :
1. Above you started with 3 green, 2 yellow and 4 blue.
(a) Give the name of the fen number that this represents.
(b) Give the fen numeral that this represents.
2. Use your work in part 2 above to complete the following equation
324f + 222f =
3. Write the fen equation that your work in part 3 above corresponds to.
Addition
De…nition
Consider how you were …rst taught to add 3 + 2: You probably did something like this: if I have
3 apples and someone gives me 2 more apples, how many apples will I have? In this example, you
are taking a set with 3 objects and a set with 2 objects and …nding the size of the union of these
two sets. We are going to use this idea to create a careful de…nition of addition of whole numbers.
De…nition 48 Let a and b be whole numbers. To …nd the sum, a + b; …nd two sets A and B such
that
1. # (A) = a;
2. # (B) = b; and
3. A \ B = ;:
Then, a + b = # (A [ B) :
Example 49 Compute 3 + 5:
Let A = f1; 2; 3g and let B = fx; y; z; w; tg :
Then # (A) = 3 and # (B) = 5 and A \ B = ;:
So, 3 + 5 = # (A [ B) = # (f1; 2; 3; x; y; z; w; tg) = 8:
Thus, 3 + 5 = 8:
Arithmetic of the Natural Numbers
30
We can use the de…nition to prove some properties of addition of whole numbers. For example,
we can use it to prove the commutative property of addition of whole numbers.
Commutative Property of Addition of Whole Numbers: For any two whole numbers a; and
b, a + b = b + a:
Proof: Let a and b be whole numbers.
Let A and B be sets so that # (A) = a; # (B) = b and A \ B = ;:
Then, by de…nition, a + b = # (A [ B) :
Also, since A \ B = ;; we have B \ A = ;: So, by de…nition b + a = # (B [ A) :
By the commutative property of unions of sets, we have A [ B = B [ A:
So,
a + b = # (A [ B)
= # (B [ A)
= b + a:
So, the commutative property of addition of whole numbers is a true property.
Algorithm ***
look at usual algorithms and need for basic addition table/facts
+fen
0fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
0fen
0fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
1fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
21fen
2fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
21fen
22fen
3fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
21fen
22fen
23fen
4fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
21fen
22fen
23fen
24fen
10fen
10fen
11fen
12fen
13fen
14fen
20fen
21fen
22fen
23fen
24fen
30fen
11fen
11fen
12fen
13fen
14fen
20fen
21fen
22fen
23fen
24fen
30fen
31fen
12fen
12fen
13fen
14fen
20fen
21fen
22fen
23fen
24fen
30fen
31fen
32fen
13fen
13fen
14fen
20fen
21fen
22fen
23fen
24fen
30fen
31fen
32fen
33fen
look at place value/expanded notation as way of building to algorithm
14fen
14fen
20fen
21fen
22fen
23fen
24fen
30fen
31fen
32fen
33fen
34fen
20fen
20fen
21fen
22fen
23fen
24fen
30fen
31fen
32fen
33fen
34fen
40fen
Arithmetic of the Natural Numbers
31
Activity –Cuisinaire Rods
Colors, from shortest to longest, are: white, red, light green, purple, yellow, dark green, black,
brown, blue, orange
1. Use Cuisenaire Rods to show:
(a) 2 + 3 = 5
(b) 4 + 2 = 6
(c) 8 + 1 = 9
2. Demonstrate 3 + 5 = 5 + 3 using Cuisenaire rods.
3. Number 2, above, is an example of what property of arithmetic?
Subtraction
A First Model –Take-Away
Our …rst model of subtraction is usually the take-away model. So, for example, to compute 5 2
we would ask, “If I have 5 of something, say cherries, and someone takes away 2 of them, how many
cherries do I have left?”
This is a very good model for subtraction and we will come back to it frequently. However, as
we will soon see, there are other models of subtraction to consider.
Arithmetic of the Natural Numbers
32
De…nition
We de…ne subtraction in terms of addition.
De…nition 50 For whole numbers a and b; we say that a
solution to a = b + c:
Example 51 Determine 5
5 2 = 3:
b = c if c is the only whole number
2: Since c = 3 is the only whole number solution to 5 = 2 + c; we have
Algorithm ***
fen
0fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
0fen
0fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
20fen
0fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
14fen
0fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
13fen
0fen
1fen
2fen
3fen
4fen
10fen
11fen
12fen
0fen
1fen
2fen
3fen
4fen
10fen
11fen
0fen
1fen
2fen
3fen
4fen
10fen
0fen
1fen
2fen
3fen
4fen
0fen
1fen
2fen
3fen
0fen
1fen
2fen
0fen
1fen
0fen
Models for Subtraction
Subtraction (word) problems can be categorized into four basic models:
1. the take-away model,
2. the missing addend model,
3. the comparison model, and
4. the geometric (or numberline) model.
Take-Away Model
Take-away model problems generally involve some collection of objects from which a some of the
objects are taken or given away.
Example 52 Jack has 7 crayons. He give 3 of his crayons to his sister Grace. How many crayons
does Jack have left?
Missing Addend Model
The missing addend model refers to the de…nition of subtraction. Remember a b = c means that
a = b + c: So, if you are trying to …nd a b; then you are trying to …nd c: In the equation a = b + c;
c is called an addend. If you don’t know c; then it would be a missing addend. In using the missing
addend model you will generally be computing a b by asking, “what do I have to add to b to get
a?”
Arithmetic of the Natural Numbers
33
Example 53 Robert has 8 toys. He wants to have 12 toys so that he can share with all of his
friends. How many more toys does Robert need to get so that he will have enough toys to share
with his friends?
In missing addend model problems there is usually a goal. In the example above, the goal was
12 toys. Robert needed to know how many toys he needed to add to his 8 toys to get to his goal
of 12 toys. That is, he needed to solve 8 + ? = 12:
This model is frequently used by cashiers. Suppose you are going to the movies and a ticket is
$7. You pay with $10. The cashier must determine (unless the cash register tells her) how much
change to give you, that is how much more than the ticket price of $7 you gave her. I refer to this
as the counting up method of subtraction because the cashier will usually count o¤ dollar bills into
your hand saying, “8, 9, 10,” and you will end up with $3 in your hand. There is no need to do
any subtraction in this case; the cashier only needs to …nd the missing addend. 7 + ? = 10
I use this method for other (non-word problem) subtraction problems as well. Suppose I want
to compute 57 23: I start at 23 and count up until I reach 27, “24, 25, 26, 27.” Then, I count up
by 10’s until I reach 57, “37, 47, 57.” Notice that I counted up by 1’s four times and by 10’s three
times, so my answer three 10’s plus four 1’s which is 34. That is, 57 23 = 34:
Comparison Model
In comparison model problems you are generally comparing two sets of objects.
Example 54 Ms. Fidler’s class has 25 students while Mr. Hoey’s class has 22 students. How
many more students does Ms. Fidler have in her class as compared to Mr. Hoey?
Note that comparison model problems are frequently written in missing addend language. For
example, we could rewrite the last sentence in the example above as “How many more students
does Mr. Hoey need to get to have as many students as Ms. Fidler does?” without changing what
the problem is asking. In this case we are asking 22 + ? = 25: You can distinguish a comparison
model problem from a missing addend model problem by noting that in a comparison problem
you will be comparing two distinct collections while in a missing addend model problem you are
generally comparing a current collection to some goal.
Geometric Model
The geometric (or numberline) model is very speci…c in that it is used when a numberline is
naturally appropriate. For example, if the numbers in the problem represent distance or length.
Geometric model problems will fall into one of the previous models as well. What distinguishes
them as primarily geometric model problems is the geometric nature of the problem, i.e. distance,
length, etc.
Example 55 In the morning Ann walks 5 kilometers due east from home to school. After school
she walks 2 kilometers west, towards home, and stops at her grandmothers house. How far is the
walk home from her grandmothers house?
Arithmetic of the Natural Numbers
34
The numberline picture above is a natural representation of the problem and we can easily see
that the distance left from grandmothers house to home is 3 kilometers.
You must be careful when describing geometric model problems to keep the motion on a straight
line. If we leave out the directions in the previous example we run into trouble. Consider the
following variation on the previous example.
Example 56 In the morning Ann walks 5 km from home to school. After school she walks 2 km
and stops at her grandmothers house. How far is the walk home from her grandmothers house?
This answer to this problem cannot be determined.
Activity –Place Value Game
1. As the number cube is rolled, …ll in each square with the single digit rolled so as to create
the LARGEST possible value. You must enter each set of digits AS IT IS ROLLED. (No fair
saving them all up and then …lling in the boxes!)
2. As the number cube is rolled, …ll in each square with the single digit rolled so as to create
the SMALLEST possible value. You must enter each set of digits AS IT IS ROLLED. (No
fair saving them all up and then …lling in the boxes!)
3. As the number cube is rolled, …ll in each square with the single digit rolled so as to create
the LARGEST possible SUM. You must enter each set of digits AS IT IS ROLLED. (No fair
saving them all up and then …lling in the boxes!)
Arithmetic of the Natural Numbers
35
4. As the number cube is rolled, …ll in each square with the single digit rolled so as to create
the SMALLEST possible POSITIVE DIFFERENCE. You must enter each set of digits AS
IT IS ROLLED. (No fair saving them all up and then …lling in the boxes!)
5. Would this game be a good classroom activity for kids? Why/Why not?
6. Explain your strategy for each part of this activity.
Activity –Base 10 Blocks
In-Class Activity - Base 10 Blocks: You will be using base 10 blocks for this activity. However,
cut up graph paper could be used as well.
units
rods
‡ats
cube
Find the Missing Numbers. Give the answer(s) as a numeral and draw a picture
of the blocks. Please give all solutions to each question.:
1. I have 5 base-ten blocks. Some are rods and some are units. Their value is less than 20. Who
am I?
2. I have 8 base-ten blocks. Some are units and some are rods. Their value is an odd number
between 50 and 60. Who am I?
3. I have 4 base-ten blocks. Some are rods and some are units. Their value is between 30 and
40. Who am I?
Arithmetic of the Natural Numbers
36
4. I have 6 base-ten blocks. Some are ‡ats, some are rods and some are units. I am a palindrome.
Who am I?
Adding and Subtracting with Blocks
1. Draw a sketch which shows how the base ten numeral 234 would be represented using base
10 pieces.
2. Suppose you have 2 ‡ats, 7 rods and 6 units and your partner has 1 ‡at, 4 rods and 9 units.
(a) Put all these pieces together and describe (sketch) the exchanges required to keep the
same total count but using the smallest number of pieces. Show each exchange.
(b) What mathematical calculation does the manipulation in part (a) represent.
(c) Explain brie‡y but clearly how the exchanges in part (a) relate to the mathematical
calculation in part (b).
3. Suppose you have 4 ‡ats and 3 units on your desk and you want to remove a count represented
by 6 rods and 7 units.
(a) Describe (sketch) the exchanges that must take place to accomplish this task. Show each
exchange.
(b) After removing the 6 rods and 7 units, what pieces are left on the desk?
(c) What mathematical calculation does the manipulation in part (a) represent.
(d) Explain brie‡y but clearly how the exchanges in part (a) relate to the mathematical
calculation in part (c).
Summary (you may discuss these questions in groups, but please don’t give group
answers; I want to know what YOU think)
1. Give a summary describing what you learned from this activity.
2. Would you use base 10 blocks to teach addition and subtraction to children? Explain your
reasons.
Converting between Fen and Ten
Now that we understand place value well, we are ready to learn how to translate a number given
in fen numerals to its base 10 (usual) notation and how to take a number given in standard (base
10) form and give its fen numeral.
Fen to Ten
Suppose we are given a number written in fen numerals. For example, 32401fen : To convert this to
base 10, we simply have to recall that the fen place value system is based on powers of fen (5 in
base 10 notation). That is, from right to left, we have the ones 50 position, the fen 51 position,
the fefen 52 position, etc. So,
32401fen = 3 54 + 2 53 + 4 52 + 0 51 + 1 50 :
Arithmetic of the Natural Numbers
37
Notice that the left-hand side of this equation is written in fen numerals, but the right-hand side
of the equation is written in base ten numerals. So, we have translated from fen to ten. Simplifying,
we get
32401fen = 3 625 + 2 125 + 4 25 + 0 5 + 1 1
= 1875 + 250 + 100 + 0 + 1
= 2226:
Ten to Fen
To take a number given in standard, base ten, notation and convert it to its base fen notation we
simply need to reverse the process above. Notice that, in the example in the previous section, if
we write 2226 as a sum of multiples of powers of 5 we get 3 54 + 2 53 + 4 52 + 0 51 + 1 50 :
The coe¢ cients of this sum, 3; 2; 4; 0 and 1; are the digits of the fen numeral 32401fen :
Example 57 Convert the number 3094 to its base fen notation.
We need to start by determining the largest power of 5 that can be subtracted from 3094:
The powers of 5 are
50 = 1
51 = 5
52 = 25
53 = 125
54 = 625
55 = 3125
..
.
So, the largest power of 5 contained in 3094 is 54 = 625:
We can take out 4 of these since 4 54 = 2500:
After taking this out we are left with 3094 4 54 = 594:
Now, we can take out some 53 ’s. We can take out 4 of these since 4 53 = 500:
After taking this out we are left with 594 500 = 94:
Now, we can take out some 52 ’s. We can take out 3 of these since 3 52 = 75:
After taking this out we are left with 94 75 = 19:
Now, we can take out some 5’s. We can take out 3 of these since 3 (5) = 15:
After taking this out we are left with 19 15 = 4:
So, we are left with 4 (1) ; that is 4 50 :
Putting all of this together we get
3094 = 4 54 + 4 53 + 3 52 + 3 (5) + 4 50 :
So, taking the coe¢ cients, we get 3094 = 44334fen. :
Example 58 Convert the number 178 to its base fen notation.
Following the process from the previous example we get
178 = 1 53 + 2 52 + 0 51 + 3 50 :
So, 178 = 1203fen :
Notice that it is important to include the fact that no 51 ’s are removed this time (i.e. 0 51 ) so
that we do not skip any place value.
Arithmetic of the Natural Numbers
38
Multiplication
De…nition
The de…nition of multiplication of whole numbers that we will be using is often referred to as
multiplication by repeated addition. You may recall have learned originally to multiply this
way. For example, to compute 3 4 you were told to add 4 to itself 3 times. That is 3 4 = 4+4+4:
De…nition 59 For any two whole numbers a and b; we de…ne a
b to itself a times, that is
a b = |b + b +{z + }b:
b to be the sum found by adding
a copies of b
Example 60 Use the de…nition above to compute each of the following.
1. 2
5 = 5 + 5 = 10
2. 5
2 = 2 + 2 + 2 + 2 + 2 = 10
3. 3
0=0+0+0
4. 0
3? In this case we need to add 3 to itself 0 times. This is interpretted to mean 0
Array Model of Multiplication ***
relate to area
use to show comm, assoc, dist properties of multiplication
expanded notation with array model
multiplying polynomials with the array model
Algorithm ***
build up using
– expanded notation
– expanded algorithm
also use array on these …rst two
– usual algorithm
Lattice Algorithm ***
Activity –Fenland Economics II
In Fenland, they have four denominations of money.
The lowest denomination is blue.
3 = 0:
Arithmetic of the Natural Numbers
39
Five blues is worth a yellow.
Five yellows is worth a green.
Five greens is worth a pink.
Bank tellers in Fenland are trained to make only one exchange at a time. That is, a teller will
make any of the four exchanges listed above, but will not combine them. For example, you can take
a green to the bank and the teller will give you …ve yellows in exchange for it. The teller WILL
NOT give you four yellows and …ve blues in exchange for a green. If you want to make such an
exchange you will have to do it one exchange at a time. That is, you can exchange your green for
…ve yellows. You can, then, exchange one of your yellows for …ve blues. You should keep track of
all exchanges you make as you work through the following activity.
1. You start with 3 pink, 1 green, 2 yellow and 2 blue. Draw a …gure representing this.
2. You want to distribute this money equally amongst four friends. Complete this process
making as few bank transactions as possible. Describe any exchanges you make along the
way to determining the answer.
3. How much does each friend receive? Do you have any money leftover?
This Fenland economics activity gives us a nice model for fen numbers. Notice that in the fen
number system
The lowest place value is the ones.
Five ones is a fen.
Five fens is a fefen.
Five fefens is a fefefen.
So, for example, 2 pink, 3 green, 1 blue models the number two fefefen, three fefen, one, that is
2301f :
1. Above you started with 3 pink, 1 green, 2 yellow and 2 blue.
(a) Give the name of the fen number that this represents.
(b) Give the fen numeral that this represents.
2. Write the fen equation that your work in part 2 above corresponds to.
Division (without remainder)
De…nition
Just as subtraction was de…ned in terms of addition, division is de…ned in terms of multiplication.
De…nition 61 For whole numbers a and b; we say that a
solution to a = b c:
b = c if c is the only whole number
Arithmetic of the Natural Numbers
40
It is important to note that c must be the only solution as we will see in one of the examples
below.
Example 62 Determine each of the following possible. If the quotient is unde…ned, explain why.
1. 5
0:
This is unde…ned since the equation 5 = 0
2. 0
0
3. 0
c has no solution.
5:
5 = 0 since c = 0 is the only solution to 0 = 5
c:
0:
This is unde…ned since the equation 0 = 0 c has more than one solution. (Note: c = 1 and
c = 2 are just two solutions since 0 = 0 1 and 0 = 0 2: There are, of course, in…nitely
many solutions to the equation 0 = 0 c:)
4. 4
3:
This is unde…ned since the equation 4 = 3
5. 8
8
c has no whole number solution.
2:
2 = 4 since c = 4 is the only solution to 8 = 2
c:
As you can see in the examples above, there are two ways for a b to be unde…ned. One
possibility is that a = b c has no solutions and the other possibility is that a = b c has more
than one solution (as with 0 0):
Recall that multiplication was de…ned as repeated addition. Since division is de…ned in terms of
multiplication and subtraction is de…ned in terms of addition, we might expect division to somehow
be related to repeated subtraction. Suppose a b = c: Then a = b c: By the commutative property
of multiplication of whole numbers, this is equivalent to a = c b: The repeated addition de…nition
of multiplication allows us to rewrite this as
a = |b + b +
{z
+ }b:
c copies of b
So, if a
b = c; then c copies of b can be subtracted from a:
Example 63 Use the repeated subtraction concept to compute 24
24
6 = 18
18
6 = 12
12
6 = 6
6
6 = 0:
So, four 6’s can be subtracted from 24. Thus, 24
6 = 4:
6:
Arithmetic of the Natural Numbers
41
Models for Division
Division (word) problems can be categorized into three basic models:
1. the missing factor model,
2. the division by grouping model, and
3. the division by sharing model.
Missing Factor Model
The missing factor model refers to the de…nition of division. Remember a b = c means that
a = b c: In the equation a = b c; c is called a factor. If you don’t know c; then it would be a
missing factor. In using the missing factor model you will generally be computing a b by asking,
“what do I have to multiply by b to get a?”
Example 64 How many hours, at $8 per hour do you have to work to earn $32?
Division by Grouping Model
The division by grouping model is also referred to as the repeated subtraction model. We discussed
the repeated subtraction concept in the previous section. Let’s look at an example.
Example 65 Ms. Johnson has 12 students in her class. She wants to put the students into groups
of 4. How many groups will there be?
To see why this model is called repeated subtraction/division by grouping. Consider how Ms.
Johnson would make her groups of 4. She would take the 12 students and “subtract”o¤ one group
of 4.
12
4=8
Leaving her with 8 students. Then, she would subtract o¤ another group of 4.
(12
4)
4=4
Leaving her with 4 students which she would put into a …nal group of 4.
Arithmetic of the Natural Numbers
42
((12
4)
4)
4=0
Leaving no students left over.
Division by Sharing Model
You may have used the division by sharing model when you did the Fenland Economics II Activity
on page 38. This is “one for you, one for me, one for you, one for me” model that kids use when
sharing something such as Halloween candy.
Example 66 Ms. Johnson has 12 students in her class. She wants to split them up into four
teams. How many students will be on each team?
How is this sharing? Think of it as Ms. Johnson “sharing” her 12 students among the four
teams.
Division with Remainder
De…nition
We know that 27 6 is unde…ned since there is no whole number c such that 27 = 6 c: However,
in school we learn to compute this and get 27 6 = 4 R: 3: That is 27 6 is 4 with a remainder of
3. In long division notation this looks like
What does this mean? To …gure this out let’s apply the division by grouping concept to the problem
27 6: In this model we start with 27 and create as many groups of 6 as possible.
Arithmetic of the Natural Numbers
43
We see that we get 4 groups of 6 with 3 left over that can’t be put into a group of 6. So, we
have 27 = 6 4 + 3:
Notice that there was nothing special about 27 and 6 in the discussion above. If you take any
numbers a and b, you can create, from a objects, groupings of b objects. You will generally have
leftover objects, as we did above. If you end up with q groupings of b objects and r things left over,
then you will have that a = bq + r: Notice that you will have fewer than b items left over (that is
r < b), since if you had b or more things left over you could pull out another grouping of b objects.
This generalized version of division with remainder is called the division algorithm.
Theorem 67 (Division Algorithm) Let a and b be whole numbers with b 6= 0: Then there exist
unique integers q and r such that a = bq + r and 0 r < b:
To …nd the q and r guaranteed by the division algorithm you simply perform the division a
q is the quotient and r is the remainder. That is, a = bq + r; if a b = q R. r:
Algorithm ***
long division algorithm
sca¤old algorithm
standard algorithm
short division algorithm
Mental Arithmetic and Estimation ***
Chapter 2 Exercises
1. Write in the words you would use to say each of the following fen numerals.
(a) 241fen
(b) 3; 201fen
(c) 12; 010fen
(d) 20; 310; 414fen
2. Write the fen numeral for each of the following fen numbers.
(a) three fefen one fen two
(b) four fefefen two fen one
(c) two fen four fefefen three
(d) one fefefen, one fen one
(e) two fen three
b:
Arithmetic of the Natural Numbers
44
3. In a base fen economy, as you would expect, they use a curious currency. Their paper money
is in four denominations. The lowest denomination is the blue. Five blues are worth one
yellow, …ve yellows in turn are worth one green and …ve greens are worth a pink.
(a) At your job in Fenland (where they use a base fen economy), you get paid twice a month.
Each time you get paid you receive one green, two yellows and three blues. Using the
smallest number of bills possible, describe your monthly pay. Describe any exchanges
you make along the way to determining the answer.
(b) You threw a big New Year’s Eve party and now you have no money. Luckily, your boss
decides to give everyone a New Year’s bonus. She gives you a bonus of two greens, one
yellow and one blue. You’re very grateful for this, since you still have one more bill
to pay from the party. The bill is for two blues and one yellow. After paying the bill,
how much do you have left (use the smallest number of bills possible in your answer).
Describe any exchanges you make along the way to determining the answer.
4. Give an example of a set which can be used to represent the number 4.
5. Use the de…nition (using set theory) of addition to show that 3 + 5 = 8:
6. What property from set theory is used to prove the commutative property of whole number
addition.
7. Recall that in the Fenland economy the lowest denomination is blue, …ve blues make a yellow,
…ve yellows make a green and …ve greens make a pink.
(a) Write the base fen name and the base fen numeral that would represent two blues, three
yellows and one pink.
(b) Now, you pay a bill for four blues. Give the base fen name and numeral for the remaining
money. (do your computations using the money–not the numerals)
(c) The bill was in error, so you get your money back and now you once again have two
blues, three yellows and one pink. You are saving up for a vacation to Ingland. The cost
of the vacation is three pinks. Give the base fen name and numeral for the amount of
money you still need in order to pay for the vacation. (make your computations using
the money–not the numerals)
(d) Now show the computations you made in parts (b) and (c) using fen numerals. Each
part was either a base fen addition or a base fen subtraction problem. Your answer
should show one calculation for part (b) and one calculation for part (c) – please label
which is which.
8. Perform each of the following base fen computations. Show any/all work. You must do all
work using base fen numerals.
(a) 12fen + 120fen
(b) 43fen + 24fen
(c) 302fen + 44fen
9. What addition fact does the Cuisenaire Rod picture below represent?
Arithmetic of the Natural Numbers
45
10. Use Cuisenaire Rods to represent each of the following:
(a) 4 + 5 = 9
(b) 3 + 3 + 2 = 8
11. Use Cuisenaire Rods and the de…nition of subtraction to explain why 8
5 = 3:
12. Perform each of the following base fen computations. Show any/all work. You must do all
work using base fen numerals.
(a) 32fen
11fen
(b) 42fen
13fen
(c) 402fen
13fen
13. Use the de…nition of subtraction to write each of the following subtraction facts as an equivalent addition fact.
(a) 8
4=3
(b) 15
9=6
(c) 24fen
(d) 331fen
13fen = 11fen
213fen = 113fen
14. Use the de…nition of subtraction of natural numbers to explain why 3
5 is unde…ned.
15. Use the de…nition of subtraction of natural numbers to explain why 5
3 = 2:
16. For each of the following word problems determine which model of subtraction the problem
best …ts into.
(a) Megan wants to paint 6 rooms in her new house. So far she has painted 2 of them. How
many more rooms does she need to paint?
(b) Mary and her brother Ken went trick-or-treating on Halloween. At the end of the night
Mary had 53 pieces of candy and Ken had 47 pieces of candy. How many more pieces
of candy does Mary have compared with Ken?
(c) On Monday, Roberto hiked 11 miles to the lake. On Friday at noon he had hiked 6 miles
back down the trail. How much farther does Roberto have to hike to get back to the
trailhead?
17. For each of the following models of subtraction, come up with a word problem that …ts the
model.
Arithmetic of the Natural Numbers
46
(a) comparison model
(b) take away model
(c) missing addend model
(d) geometric model
18. .
(a) List (in words) any/all exchanges necessary to compute 327
49:
(b) Use the standard algorithm (i.e. the one you learned in school) to compute 327
49:
(c) Explain where each of the exchanges you described in part a show up in your computation
in part b.
19. Convert each of the following base fen numerals into an equivalent base ten numeral. Show
any relevant work.
(a) 324fen
(b) 12; 003fen
(c) 3; 024; 414fen
20. Convert each of the following base ten numerals into an equivalent base fen numeral.
(a) 200
(b) 117
(c) 20; 346
21. State each of the following properties of whole numbers.
(a) Commutative property of addition of whole numbers.
(b) Associative property of multiplication of whole numbers.
(c) Distributive property of multiplication over addition of whole numbers.
22. Use the de…nition of multiplication (as repeated addition) to compute each of the following.
Make sure that you show your work so that I can tell that you’re using the de…nition.
(a) 4
9
(b) 9
4
(c) 2fen
4fen
(d) 3fen
10fen
23. .
(a) Use the de…nition of multiplication (as repeated addition) to compute 15
3:
(b) Use the de…nition of multiplication (as repeated addition) to compute 3
15:
(c) Which was easier to compute?
Arithmetic of the Natural Numbers
47
(d) If you had to make the computation mentally (i.e. in your head without paper and
pencil or a calculator) which form of the problem (15 3 or 3 15) would you choose
and why?
(e) What property of whole numbers tells us that the computations from parts a and b or
this problem will yield the same result.
24. Use the array model of multiplication to show/explain why 4
9=9
4:
25. Complete the fen multiplication table below:
fen
0fen
1fen
2fen
3fen
4fen
0fen
1fen
2fen
2fen
0fen
3fen
22fen
4fen
4fen
26. Use the lattice algorithm to complete each of the following computations.
(a) 312
(b) 312fen
42
42fen
27. In each of the following, determine which property of whole numbers justi…es the equality.
(a) 4 9 = 9 4
(b) 4 (6 + 2) = 4 6 + 4 2
(c) 3 + 8 = 8 + 3
(d) 12 + (3 + 2) = (12 + 3) + 2
(e) 5 (9 11) = (5 9) 11
(f) 7 3 + 7 8 = 7 (3 + 8)
28. For each of the following subtraction problems (taken from Exploring Mathematics, Grade 2,
Scott Foresman and Company), determine which model of subtraction is being used. Then,
solve the problem.
(a) Amy’s book has 124 pages. Steve’s book has 133 pages. How many more pages are in
Steve’s book than Amy’s?
(b) Jodi’s book has 75 pages. On Monday she read 32 pages. How many pages did she have
left to read?
(c) Lee took 65c/ to the swimming pool. He spent 33c/ for a waterslide ticket. How much
did Lee have left?
Arithmetic of the Natural Numbers
48
(d) The second graders have 38 more books than the …rst graders. The second graders have
87 books. How many books do the …rst graders have?
29. Recall that in the Fenland economy the smallest denomination is blue, …ve blues is equivalent
to one yellow, …ve yellows is equivalent to one green and …ve greens is equivalent to one pink.
(a) You have 1 blue, 4 yellow, 1 green and 2 pink. This money needs to be distributed
evenly amongst 4 people. Determine how much each person gets. (give your answer in
terms of the money, not as a fen numeral)
(b) Show all bank transactions (exchanges) necessary to complete the distribution. That is,
go through the process used to determine the answer to part (a) one step at a time.
(c) Use fen numerals to describe the division problem that this problem represents.
(d) What model of division does this problem …t into?
30. For each of the following models of division, come up with a word problem that …ts the model.
(a) division by sharing model
(b) missing factor model
(c) division by grouping model
31. Explain why 0
32. Explain why 13
13 = 0:
0 is unde…ned.
33. Use the long division algorithm to compute 952
34. Use the sca¤old algorithm to compute 952
6:
6:
35. Use the short division algorithm to compute 952
6:
36. Place the digits 3; 6; 7 and 8 in the boxes below (using each digit exactly once) to obtain...
(a) the greatest quotient
(b) the least quotient
37. .
(a) Compute 214fen
numerals).
3fen using whichever division algorithm you prefer. (use only fen
(b) Use the result in part (a) to …ll in the blanks below:
214fen = 3fen
+
38. What do you consider to be the “best” algorithm for multiplication that we have studied.
Explain the reasoning behind your choice.
Arithmetic of the Natural Numbers
49
39. Supply reasons for each of the …rst three steps given below
(525 + 37) + 75 = 525 + (37 + 75)
= 525 + (75 + 37)
= (525 + 75) + 37
= 600 + 37
= 637
40. Suppose x and y are positive whole numbers. If x is greater than y and you estimate x y
by rounding x up and y down will your estimate always be too high or always be too low or
could it be either (i.e. sometimes too low, sometimes too high)? Explain.
41. Classroom situations:
(a) A student asks if 39 + 41 = 40 + 40; is it true that 39 41 = 40 40: How do you reply?
(b) Another student says that he knows that 39 41 =
6 40 40 but he found that 39 41 =
40 40 1. He also found that 49 51 = 50 50 1: He wants to know if this pattern
continues. How would you respond?
50
Arithmetic of the Natural Numbers
Chapter 3
Number Theory
Divisibility
Much of number theory centers around the notion of divisibility.
De…nition 68 Let a and b be integers. We say that b divides a; denoted bja if there is a unique
integer q such that a = bq: In this situation we also say
1. b is a factor of a
2. b is a divisor of a
3. a is a multiple of b
Recall that when you divide an integer a by another integer b; you get a quotient, q; and a
remainder, r: These values are related by the equation a = bq + r: So, if bja; we are getting a = bq;
i.e. r = 0: So, bja means that there is no remainder when a is divided by b: That is, that a b
is an integer. Even though there is this relationship between the notion of divides and division,
this is not the de…nition. When working with this concept, it is important to remember that the
de…nition is as given at the start of this section.
Example 69 True or false: 6j24:
This is true since 24 = 6 4:
Example 70 True or false: 24 is a multiple of 5:
This is false since there is no integer q such that 24 = 5q:
One way to visualize the notion of divisibility (at least for positive values of a and b) is by using
the array model of multiplication. If b divides a; then we have a = bq for some integer q (note that
if a and b are positive, then q is also positive). So, there is a b q array using a squares. So, we
can determine that b divides a; if we can make an array with b rows using exactly a squares.
51
Number Theory
52
Example 71 Use the array model to show that 6j24:
The array below has 6 rows and uses 24 squares. In fact, it shows 24 = 6 4: Thus, 6j24:
Example 72 Use the array model to show that 24 is not a multiple of 5:
If 24 were a multiple of 5, then there would be an array with 5 rows using 24 squares. If I start
putting my 24 squares into columns of 5, I will get stuck after I have used 20 of my squares to get
and have four boxes left over that can’t be used to build another column of 5.
Thus, 24 is not a multiple of 5.
De…nition 73 The natural numbers can be classi…ed as prime, composite or unit.
A natural number with exactly two positive divisors (itself and 1) is a prime number.
A natural number with more than two positive divisors is said to be composite.
The natural number 1 is called a unit.
Primes
There are in…nitely many primes
Euclid gave the …rst known proof that there are in…nitely many primes (ca. 300 B.C.). His
argument went something like this. Suppose that you have a list of primes, p1 ; p2 ; : : : ; pr . Multiply
them together and then add 1. Call this number N: So, N = p1 p2
pr + 1:. Either N is prime
or, by the FTA, it can be factored into primes. If N is prime, then N is a prime that is not on the
list because it is larger than any of the primes on the list. If N is not prime, then N must have a
prime factor q: Notice that q cannot be on our original list of primes since q divides N evenly, but
when N is divided by any of the original primes, pi ; a remainder of 1 is left.
Number Theory
53
Determining whether or not a number is prime
To be able to determine whether or not a speci…c number is prime, it is helpful to know some
relatively small primes. The sieve of Eratosthenes (ca. 200 B.C.) will give us a way to determine
these. We will use the sieve of Eratosthenes below to determine all primes between 1 and 200: The
technique for using the sieve of Eratosthenes is as follows:
1. Write a list of numbers from 2 to the largest number you want to test for primality (in this
case 200).
Note: you should cross out the number 1 on the sieve below, since we are only supposed
to list the numbers from 2 to 200.
2. Circle the number 2, the …rst prime number.
3. Cross o¤ all multiples of 2 from the sieve.
4. The …rst number not circled or crossed o¤ remaining in the sieve is a prime number. Circle
this number.
5. Cross o¤ all multiples of this number.
6. Repeat steps 4 through 6 until all numbers in the sieve have either been circled or crossed o¤.
7. The circled numbers are the only primes on the list.
Number Theory
54
Now, suppose we want to know whether a number, such as 439 is prime. One option would be
to start a new sieve with the numbers from 2 to 439 on it, but that would be a lot of work simply
to answer this question.
Another option would be to go back to the de…nition. If 439 is not prime, then it is composite
and that would mean that 439 has at least one other divisor other than 1 and itself, 439: So, we
could
check for divisors of 439 from among the integers between 2 and 438.
That would still be a lot of work. Can we simplify things?
Number Theory
55
First notice that if 439 is not prime, then it must have a prime divisor, so we only need to
check for divisors of 439 from among the primes between 2 and 438.
To do that we would need to know all of the primes between 2 and 438. We know the primes
between 2 and 200, but we don’t know the ones between 201 and 438. Can we simplify further?
Suppose p is a divisor of 439, other than 1 or 439, then pq = 439 for some integer q: Since p is
1
439
439
not equal to 1 or 439, q is also not equal to 1 or 439. So, q 2: So, 1q
2 ; so p = q
2 = 219:5:
So, if 439 is not prime then it must have a prime divisor less than or equal to 219:5: So, we only
need to
check for divisors of 439 from among the primes between 2 and 219.
This is much better since we already know the primes between 2 and 200, but this is still a
lot to check. Remember that we found 46 primes between 2 and 200. So, we have more than 46
divisors to check. Can we improve this any more?
Suppose p is a divisor of 439. Then pq = 439 for some
p integer q: Notice that at least one of
p the
two numbers,
p
and
q,
must
be
less
than
or
equal
to
439:
To
see
why,
suppose
not,
so
p
>
439
p
p
p
p
p
and q > 439: Then pq > 439 439: But pq = 439 and 439 439 = 439; so this is saying
439 > 439
p which is clearly
p false. Therefore, it must be that one of the two number is less than or
equal to 439: Notice 439 20:95, so we only need to
check for divisors of 439 from among the primes between 2 and 20. That is,
check for divisors of 439 from among 2, 3, 5, 7, 11, 13, 17, 19.
Let’s check:
Does 2 divide 439? No.
Does 3 divide 439? No.
Does 5 divide 439? No.
Does 7 divide 439? No.
Does 11 divide 439? No.
Does 13 divide 439? No.
Does 17 divide 439? No.
Does 19 divide 439? No.
Therefore, since 439 does not have any divisors form among 2, 3, 5, 7, 11, 13, 17, 19, we know
that 439 is not composite. Therefore, 439 is prime.
Example 74 Determine whether or not 3127 is prime.
Based
on the work above we only need to check for divisors from among the primes between 2
p
and 3127 55:9: So, we need to check to see if any of the following are divisors of 3127 :
2; 3; 5; 7; 11; 13; 17; 19; 23; 29; 31; 37; 41; 43; 47; 53
Does
Does
Does
Does
Does
Does
2 divide 3127? No.
3 divide 3127? No.
5 divide 3127? No.
7 divide 3127? No.
11 divide 3127? No.
13 divide 3127? No.
Number Theory
56
Does 17 divide 3127? No.
Does 19 divide 3127? No.
Does 23 divide 3127? No.
Does 29 divide 3127? No.
Does 31 divide 3127? No.
Does 37 divide 3127? No.
Does 41 divide 3127? No.
Does 43 divide 3127? No.
Does 47 divide 3127? No.
Does 53 divide 3127? Yes, since 3127 = 53 59:
Therefore, 3127 has at least 3 divisors: 1, 53 and 3127. So, 3127 is not prime.
In the two preceeding examples we have been making use of the following lemma.
Lemma 75 If a number n is composite, then there is a prime p such that pjn and p
p
n:
p
Notice that this lemma does not say that all divisors of n are less than or equal to n; it simply
p
says that if any prime divisors exist, then at least one of them must be less than or equal to n:
Notice, for example, that in thep example above 3127 is a composite and has a prime divisor,
53, which is less
p than or equal to 3127: However, 3127 has another prime divisor, 59, which is
greater than 3127:
Largest Known Prime
As of 17 January 2007, the largest know prime is 232582657 1: This prime has 9,808,358 digits. If you want to see whether any larger primes have been found since January 17th, check out
www.utm.edu/research/primes/largest.html. This website is a good resource for general information about prime numbers.
Fundamental Theorem of Arithmetic
Theorem 76 Every natural number (other than the number 1) can be expressed as a product of
primes in exactly one way (up to order).
This theorem says that any natural number (other than the number 1) can be factored into
primes. Furthermore, if two people both factor the number into primes, they will get the same
result other than the fact that they might have the primes listed in a di¤erent order. For example,
if two people are factoring the number 12 into primes, one person might get 12 = 3 2 2 while
another person gets 12 = 2 3 2: The theorem states that everyone will get two 2’s and one 3 when
factoring 12 into primes.
How does one …nd the prime factorization of a number? The most common method is to use a
factor tree. Recall that in a factor tree you start with your number and work downward factoring
as you go. You stop when all of the factors are primes.
Example 77 Use a factor tree to …nd the prime factorization of 1200.
Number Theory
57
So, 2 2 3 2 5 2 5 is the prime factorization of 1200.
To make it easier to read, we usually write the prime factorization of a number in prime power
representation. When writing a number in prime power representation you put the primes in the
factorization in order from smallest to largest and use exponents to simplify, e.g. write 23 instead
of 2 2 2:
Example 78 Determine the prime power representation of 1200.
In the previous example we saw that the prime factorization of 1200 contains four 2’s, one 3
and two 5’s. So, the prime power representation of 1200 is
1200 = 24 3 52 :
Number of Divisors ***
Lemma 79 A number n written in prime power representation as n = pa11 pa22
(a1 + 1) (a2 + 1)
pakk has
(ak + 1)
positive divisors.
Tests for Divisibility
Divisibility
Divisibility
Divisibility
Divisibility
Divisibility
by 2, 5 and 10 ***
by 3 and 9 ***
by 11 ***
by 2r ***
Rules for Composites ***
Greatest Common Divisor (GCD) and Least Common
Multiple (LCM)
De…nitions
The de…nitions of greatest common divisor and least common multiple are very straightforward.
We already know what the words greatest, least, common, divisor and multiple mean, so we just
Number Theory
58
have to put them together into some de…nitions.
De…nition 80 For two integers a and b; the greatest common divisor of a and b; denoted
gcd (a; b) ; is the largest natural number which divides both a and b:
De…nition 81 For two integers a and b; the least common multiple of a and b; denoted lcm (a; b),
is the smallest natural number which is a multiple of both a and b:
Set Intersection Method
The set intersection method is a precise version of what is usually taught as a …rst method for
computing gcd’s and lcm’s. It is nothing more than an application of the de…nitions.
To use the set intersection method to compute gcd (a; b) you start by …nding the set of all
divisors of a; denoted D (a) ; and the set of all divisors of b; denoted D (b) : Then D (a) \ D (b) will
contain those elements common to both D (a) and D (b) ; that is numbers that are divisors of a
and divisors of b; common divisors of a and b: So, the largest element of D (a) \ D (b) will be the
greatest common divisor of a and b; gcd (a; b) :
Example 82 Use the set intersection method to …nd gcd (14; 22) :
D (14) = f1; 2; 7; 14g
D (22) = f1; 2; 11; 22g
D (14) \ D (22) = f1; 2g
gcd (14; 22) = 2:
Similarly, to …nd lcm (a; b) ; we should look at the set of all multiples of a; M (a) ; and the set of
all multiples of b; M (b) : Then M (a) \ M (b) will be the set of all common multiples of a and b: So,
the smallest element of M (a) \ M (b) will the be the least common multiple of a and b; lcm (a; b) :
Example 83 Use the set intersection method to …nd lcm (6; 8) :
M (6) = f6; 12; 18; 24; 30; 36; 42; 48; : : :g
M (8) = f8; 16; 24; 32; 40; 48; 56; 64; : : :g
M (6) \ M (8) = f24; 48; : : :g
lcm (6; 8) = 24:
The set intersection method is a good …rst method because it reinforces the de…nition of gcd
and lcm : However, it is unreasonable to use when the numbers involved are large. (If you don’t
believe me, try using this method to …nd gcd (504; 3675) and lcm (504; 3675) :)
Prime Power Representation Method ***
Euclidean Algorithm Method ***
The Euclidean algorithm is generally the best method for determining gcd (a; b) when the numbers
a and b are relatively large. It does not require you to be able to factor the numbers a and b: The
following theorem is why the Euclidean algorithm works.
Theorem 84 If a = bq + r where a; b; q and r are integers, then gcd (a; b) = gcd (b; r) :
Number Theory
59
Proof. To prove this we will show that D (a) \ D (b) = D (b) \ D (r) ; so that the largest element
of D (a) \ D (b) ; i.e. gcd (a; b), is equal to the largest element of D (b) \ D (r) ; i.e. gcd (b; r) : So,
we need to show that the common divisors of a and b are precisely the common divisors of b and r:
Suppose d is a common divisor of a and b: Then a = dq1 and b = dq2 for some integers q1 and q2 :
So, since a = bq + r; we have dq1 = (dq2 ) q + r: Solving for r we get r = dq1 (dq2 ) q = d (q1 q2 q) :
So, d is a divisor of r: We assumed that d was a divisor of b: Thus, d is a divisor of both b and r:
So, d is a common divisor of both b and r:
Similarly, we can show that a common divisor of b and r must be a common divisor of a and b:
Therefore, gcd (a; b) = gcd (b; r) :
Activity –Interesting Numbers
The lyrics of an old song assert that “One is the loneliest number.” This makes 1 interesting, but
all numbers have some property that makes them interesting.
1. 121— I’m a perfect square, since I’m equal to 11x11, but there is something else interesting
about me. What is it?
2. Find three more perfect squares.
3. Find three palindromes. (A palindrome is a number that is unchanged when its digits are
reversed.)
4. 64— I’m equal to 4x4x4. This means I’m a perfect cube. Find something else that is interesting
about me.
5. Find three more perfect cubes.
6. What is the smallest multi-digit number that is both a perfect cube and a palindrome?
7. 13— I’m prime since my only factors are 1 and 13, but I have another property that not all
primes have. What is it? HINT: Reverse my digits.
8. How many palindromes between 100 and 200 are primes?
9. Find three prime numbers such that when their digits are reversed, the result is also a prime
number.
INTEREST RATING:
Prime Number
Perfect Cube
Perfect Square
Sum of Digits Greater than 14
Even Number
Odd Number
Each Factor
15 points
10 points
7 points
4 points
3 points
2 points
1 point
Number Theory
60
Example: 169
Perfect Square (169 = 13
13) . . . 7 points
Sum of Digits Greater Than 14 (1 + 6 + 9 = 16 > 14) . . . 4 points
Odd Number . . . 2 points
Three Factors (1; 13; 169) . . . 3 points
Interest Rating = 7 + 4 + 2 + 3 = 16 points
1. Choose any three-digit number. Find its interest rating using the scale.
2. Can you …nd a three-digit number with an interest rating greater than 30 points? Explain.
3. What is the greatest possible interest rating for a three-digit prime number? Explain.
4. Explain why a number greater than 1 that is both a perfect cube and a perfect square would
have an interest rating of at least 24 points.
5. Are there any three-digit numbers that are both perfect cubes and perfect squares? If so,
what are their interest ratings?
6. Find the highest possible interest rating for a three digit number. Explain.
Chapter 3 Exercises
1. Fill in the missing numbers in the following factor tree.
2. Classify each of the following as true or false. Justify your answer using the de…nition of
divisibility.
(a) 11 is a factor of 189
(b) 1001 is a multiple of 13
(c) 6j30
(d) 24 is divisible by 6
(e) 12 is a multiple of 36
(f) 6 divides 2
Number Theory
61
3. Draw an array for each of the following facts:
(a) 4 divides 12
(b) 6 is divisible by 2
(c) 2j8
(d) 20 is a multiple of 5
(e) 3 is a factor of 15
4. .
(a) When the U.S. ‡ag had 48 stars, the stars formed a 6 8 rectangular array. In what
other rectangular arrays could they have been arranged?
(b) How many rectangular arrays of stars could there be if there were only 47 states?
5. Brie‡y explain why 23 32 253 is not a prime factorization and …nd the prime factorization
of this number.
6. Produce the smallest positive number that is divisible by 2, 3, 4, 5, 6, 7, 8, 9, 10 and 11.
7. Find the largest 4-digit number that has exactly three factors.
8. Find a number that has exactly six prime factors.
9. Use a factor tree to …nd the prime factorization for each of the following:
(a) 504
(b) 2475
(c) 11,250
10. Use the results from problem 9 to determine the number of divisors of each of the following:
(a) 504
(b) 2475
(c) 11,250
11. List all divisors of each of the following:
(a) 100
(b) 363
(c) 232
(d) 5,929
12. Based on lemma 75, what is the largest prime you must consider to test whether 5669 is
prime?
13. The prime numbers 11 and 13 are called twin primes because they di¤er by 2. Find all twin
primes less than 200. (you may …nd it helpful to use the Sieve of Eratosthenes)
Number Theory
62
14. Is it possible to …nd natural numbers a; b and c such that 2a 3b = 5c ? Why or why not?
15. Is 3111 a prime? Prove your answer.
16. Find two whole numbers x and y such that xy = 1; 000; 000 and neither x nor y contains any
zeros as digits.
17. Fill in each blank space with a single digit that makes the corresponding statement true. Find
all possible answers
(a) 3j83_51
(b) 11j8_691
18. If 42jn; what other positive integers necessarily divide n?
19. In order to test for divisibility by 12, one student checked to determine divisibility by 3 and 4,
while another checked for divisibility by 2 and 6. Are both students using a correct approach
to divisibility by 12? Why or why not?
20. Without using a calculator, test each of the following numbers for divisibility by 2; 3, 4, 5, 6,
8, 9, 10 and 11.
(a) 746,988
(b) 15,810
21. When the two missing digits in the following number are replaced, then number is divisible
by 99. What is the number?
85__1
22. Classify each of the following as always true or sometimes false:
(a) If every digit of a number is divisible by 3, the number itself is divisible by 3.
(b) If a number is divisible by 3, then every digit of the number is divisible by 3.
(c) If a number is divisible by 6, then it is divisible by 2 and by 3.
(d) If a number is divisible by 2 and by 3, then it is divisible by 6.
(e) If a number is divisible by 2 and 4, then it is divisible by 8.
(f) If a number is divisible by 8, then it is divisible by 2 and by 4.
23. The numbers x and y are divisible by 5.
(a) Is xy divisible by 5? Why?
(b) Is x
y divisible by 5? Why?
24. For each of the following determine all values for the missing digit in the number below so
that the number is divisible by
527; 4_2
(a) 2
(b) 3
Number Theory
63
(c) 4
(d) 9
(e) 11
25. Leap years occur in years that are divisible by 4. However, if the year ends in two zeros,
in order for the year to be a leap year, it must be divisible by 400. Determine which of the
following are leap years:
(a) 1775
(b) 1986
(c) 2000
(d) 2024
26. Find the GCD and LCM for each of the following using the intersection-of-sets method:
(a) 18 and 10
(b) 24 and 36
(c) 8, 24 and 52
(d) 7 and 9
27. Find the GCD and the LCM for each of the following using the prime factorization method:
(a) 132 and 504
(b) 65 and 1690
(c) 900, 96 and 630
(d) 108 and 360
(e) 11 and 19
(f) 625, 750 and 1000
28. Find the GCD for each of the following using the Euclidean algorithm. Then, use the theorem
relating GCD’s and LCM’s to compute the LCM of each pair:
(a) 220 and 2924
(b) 14,595 and 10,856
(c) 122,368 and 123,152
29. Bill and Sue both work at night. Bill has every sixth night o¤ and Sue has every eighth night
o¤. If they are both o¤ tonight, how many nights will it be befor ethey are both o¤ again?
30. Jackie spent the same amount of money on cassette tapes that she did on compact discs. If
tapes cost $12 and CDs $16, what is the least amount she could have spent on each?
31. By selling cookies at 24c/ each, José made enough money to buy several cans of pop costing
45c/ per can. If he had no money left over after buying the pop, what is the least number of
cookies he could have sold?
Number Theory
64
32. At the Party Store, paper plates come in packages of 30, paper cups in packages of 15, and
napkins in packages of 20. What is the least number of plates, cups, and napkins that can be
purchased so that there is an equal number of each?
33. Find all natural numbers x such that gcd (25; x) = 1 and 1
x
25:
34. Can two numbers have a greatest common multiple? Explain.
35. Is it true that gcd (a; b; c) lcm (a; b; c) = abc? Explain your answer.
36. Suppose that gcd (a; b; c) = 1: Is it necessarily true that gcd (a; b) = 1 and gcd (b; c) = 1?
Explain your reasoning.
37. Suppose gcd (a; b) = 2 and gcd (b; c) = 2: Does that always imply that gcd (a; b; c) = 2? Justify
your answer.
38. Given that a and b are natural numbers with ab = 1; 515; 025; 512 = 23 33 73 112 132 and
gcd (a; b) = 14;
(a) Determine lcm (a; b) without …rst …nding a and b:
(b) Is it possible to determine a and b? Explain.
Chapter 4
The Integers
Opposites
Up to this point we have been studying the whole numbers. The whole numbers form a useful
number system, however, this system has some de…ciencies. For example, using only the whole
numbers, we cannot solve a simple equation such as 4 + x = 3: This is related to the fact that
subtraction is not closed on the whole numbers. Recall (see page 16) that the closure property of
subtraction of whole numbers is: for any two whole numbers a and b; a b is a whole number. This
is a false property since 3 and 4 are whole numbers, but 3 4 is not a whole number. Once we
expand our system of numbers to the integers, we will see that the closure property of subtraction
of integers is true.
The way to progress from the whole numbers to the integers is by creating additive inverses for
each whole number. Let’s start with some de…nitions.
De…nition 85 A number a is called an additive identity if a + x = x for every number x:
The number 0 has the property that 0 + x = x for every number x; so 0 is an additive identity.
So, the whole numbers have an additive identity.
De…nition 86 For any number x; the number y is called the additive inverse of x if x + y = 0:
Notice that the number 3 is a whole number, but there is no whole number y such that 3+y = 0:
So, 3 has no additive inverse in the whole numbers. However, the integer 3 is an additive inverse
for 3 since 3 + ( 3) = 0: We will refer to the additive inverse of a number as the opposite of that
number. So, 3 is the opposite of 3: Here are a few other ways to state the de…nition given above
for additive inverse/opposite.
De…nition 87 For any number x; the number y is called the opposite of x; if x + y = 0:
De…nition 88 The opposite of a number x is the number y that when added to x gives a sum of
0.
65
The Integers
66
De…nition 89 For any number x; the equation x + y = 0 tells us that y is the opposite of x:
The notation that we will use for the opposite of a number x is x: Note that this should be
read “opposite of x,” NOT as “negative x” as the opposite of a number need not be negative. For
example, since 3 + 3 = 0; we have that 3 is the opposite of 3: So, ( 3) = 3 (read this as “the
opposite of negative 3 is 3,”or as “the opposite of the opposite of 3 is 3”). It is permissible to read
3 as “negative 3”since it truly is a negative number. It may also be read as “opposite of 3”, since
it is also the opposite of 3.
The set of integers (remember the notation for the set of integers is Z) can be partitioned into
three subsets:
1. the set of positive integers (this is just the set of natural numbers, N)
2. the set of negative integers (these are the opposites of the natural numbers)
3. the set containing the number 0. It is important to note here that 0 is neither positive nor
negative.
Using the notation x for the opposite of x; we can restate the de…nitions above as follows.
De…nition 90 The opposite of a number x is the number
x such that x + ( x) = 0:
De…nition 91 x + y = 0 () y = x (read this as “x plus y is equal to 0 if and only if y is equal
to the opposite of x:”)
Representations of the Integers
Most of the common representations of integers involve using two-sided objects. One side represents
positive and the other, negative. Colored counters are sold at teacher supply stores and are either
disk or bean shaped with one side red (representing negative) and the other side white or yellow
(representing positive). You can achieve the same e¤ect by using pennies (heads for positive, tails
for negative). You will get some practice with these manipulatives as a representation for the
integers in the activity following this section.
A similar representation is the charged particle representation. This is essentially the same idea
as the counters, except that you do not use a hands-on manipulative. Instead you use positive
particles, drawn as
, and negative particles, drawn as
An example of how to use these is as follows.
.
Example 92 What integer is represented by the following collection of charged particles?
The Integers
67
Each pair of one positive particle and one negative particle contributes nothing to the overall
value (since a negative particle is the opposite of a positive particle and a number plus its opposite
is zero).
So, after pairing o¤ positive/negative pairs, we are left with
so there are two negative particles left. Thus, this collection of charged particles represents
2:
The idea of charged particles works well if the students are also studying atoms in their science
class. For example, in the previous example, I could have restated the question as, “What is the
charge of the ion containing two protons and four electrons?”
Activity –Colored Counters
PURPOSE: Investigate a concrete model for integers and use it to discover algorithms for
integer addition.
MATERIALS: A paper cup (optional), 6 pennies, a game marker for each player and a game
board (a number line showing the integer values from 10 to 10).
First A Game–The Rules:
The Integers
68
At the beginning of the game, each player places a game marker on zero on the game board.
Players alternate turns.
On your turn place 6 pennies in the cup (or in your hands), cover the opening with your
hand, shake the cup thoroughly, and drop the pennies onto the table. Each HEAD means
you move your marker to the right one unit; each TAIL means move it to the left one unit.
The …rst player to reach 10 or 10 on the game board is the winner. If there is no winner
after …ve turns, the player closes to 10 or 10 wins.
Play the game. When you have …nished, answer the following questions. Explanations are not necessary for these 4 questions, but please discuss the answers in your
group before moving on to the rest of this assignment.
1. Did you …nd a way to quickly determine where to place your game piece after the coins were
tossed?
2. If you were to represent the number of HEADs with an integer, would you use a positive or
a negative integer?
3. Would you use a positive or a negative integer to represent the number of TAILs?
4. Did your game piece ever end up on an odd number?
Recall that colored counters are disks which are white on one side (representing positive, like
HEADs) and red on the other side (representing negative, like TAILs). The game you just played
gave you an idea of how colored counters (or pennies) can be used to represent integers.
1. Use markers (you can use as many as you want for this) to construct two di¤erent representations for each of the following integers: 4; 3; 0:
2. Let A be the set of all integers that can be represented using exactly …ve counters.
(a) Give A in roster notation.
(b) Determine # (A) :
3. Is it possible to get a representation of the integer 1 using exactly four counters? Explain.
Colored counters can also be used to model addition of integers. For example, a toss in the
game of 2 white and 4 red is the same as adding 2 and 4:
The Integers
69
1. Consider the diagram above
(a) Why do the paired counters "cancel" each other out?
(b) If you tossed this combination of counters, how would you move your game piece?
(c) What integer is represented by the combination of counters?
(d) Complete the equation 2 + ( 4) =
2. Use colored counters to determine each of the following.
(a) 2 + 4
(b) 2 + ( 3)
(c) ( 2) + ( 4)
(d) ( 2) + 3
We would like to use counters to model subtraction. One of our best models for subtraction
of whole numbers was the take-away model. Recall that to model 7 4 we would place 7 markers
on the table and take away 4 of them leaving us with the answer, 3. We would like to extend this
model so that it would work as a model for subtraction of integers.
1. Use your counters and the take away model to compute each of the following di¤erences.
Make a sketch of your work. Do not turn these into addition problems!!!! That is, when
asked to compute ( 3) 4; do NOT replace the problem with ( 3) + ( 4) ; instead you
must determine the answer to the following question “if I start with (markers representing)
negative three and take away (markers representing) positive four, what am I left with?”
(a) 4
2
(b) 4
5
(c) ( 3)
4
(d) ( 5)
( 2)
Subtraction as Addition
The representation of integers using colored counters (or pennies or charged particles) is useful for
developing intuition and understanding regarding this arithmetic. However, we have given a careful
de…nition of the negative integers as opposites to the natural numbers, so we need to prove that
the results we have been getting when adding and subtracting integers are true. We start with a
basic fact about taking the opposite of the opposite of a number.
Lemma 93 For any integer x;
( x) = x:
Proof. By de…nition of the opposite of x; x, we know that x + ( x) = 0:
So, by the commutative property of addition of integers, ( x) + x = 0:
So, x is a number that when added to x gives 0. That means that x is the opposite of x:
So, x = ( x) :
The Integers
70
We could give justi…cation to this lemma by using the colored counters as well. Notice that,
with the colored counters, the way to …nd the opposite of a number is to take a representation of
the number and then ‡ip over the counters. So, to …nd ( x), you take a representation of x:
Then, you ‡ip over the counters to get a representation of x: Then, you ‡ip the counters again
to get ( x) : Now, you’ve ‡ipped the counters twice, so you’re back where you started. That is
( x) = x:
Remember that our goal is to prove addition and subtraction facts integers. That is, we want
to be able to prove things like 7 + ( 4) = 3 and 5 7 = 12: We have seen in our past experience
with this mathematics that 5 7 = 5 + ( 7) : In fact, most of us have been taught to compute
all subtraction problem involving integers by rewriting the problem appropriately as an addition
problem. If we can prove that this is legitimate, then we only need to learn how to prove addition
facts, subtraction facts will simply be turned into addition facts. To do this we need to prove the
following general fact which I will call the subtraction as addition lemma.
Lemma 94 For any integers a and b;
a
b=a+
b :
Proof. Let a and b be integers.
Then, by de…nition, a b = c means c is the only solution to a = b + c:
We will show that c = a + ( b) :
Consider b + (a + ( b)) :
b+ a+
b
= b+
b + a by commutative property of addition of integers
=
b
b+
+ a by associative property of addition of integers
= 0 + a by de…nition of opposites
= a by the additive identity property of 0.
Thus, b + (a + ( b)) = a: So, a + ( b) is a solution to b + c = a:
Therefore, a b = a + ( b) :
So, using this lemma, we only need to know how to add integers.
Example 95
3
4 = 3 + ( 4)
Example 96 2 ( 1) = 2 + ( 1) = 2 + 1
Note, this lemma tells us that 2 ( 1) = 2+ ( 1) : The previous lemma tells us that
Putting these together gives us 2 ( 1) = 2 + ( 1) = 2 + 1:
( 1) = 1:
The colored counters were a good model for addition, but now we need to prove that what we
were doing was correct.
Addition of Integers
When proving addition facts involving integers you must be careful to only use things that you
have already proven. You may use properties of addition of integers (commutative and associative).
You may use the de…nition of opposites. You may use the additive identity property of zero, that
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71
is 0 + x = x = x + 0 for any value of x: That is all that may be used. You may NOT use facts
such as 3 + ( 4) = 1 since that is precisely the kind of fact that we are trying to prove here. You
may, however use facts such as 3 + 4 = 7 since that is a result from whole number arithmetic and
we have already been through the proofs of those types of facts.
We are going to be trying to prove facts of the form a + b = c where a; b and c are integers.
There are two possibilities: c is a whole number, c is a negative integer. The approach for the proof
will be somewhat di¤erent depending on which case you are in.
If the result, c, is a whole number, then the proof is usually a straightforward proof where you
work with a + b until you are able to show that it is equal to c:
Example 97 Prove 7 + ( 4) = 3:
7+
4
= (3 + 4) +
4 by whole number arithmetic
4
= 3+ 4+
by the associative property of addition of integers
= 3 + 0 by de…nition of opposites
= 3 by the additive identity property of 0.
Therefore, 7 + ( 4) = 3:
Notice that, in a proof, you must give a justi…cation for each claim you make. Also, you should
always conclude the proof by restating what you have proven.
Example 98 Prove ( 4) + 9 = 5:
4 +9 =
=
=
4 + (4 + 5) by whole number arithmetic
4 + 4 + 5 by the associative property of addition of integers
4+
4
+ 5 by the commutative property of addition of integers
= 0 + 5 by de…nition of opposites
= 5 by the additive identity property of 0.
Therefore, ( 4) + 9 = 5:
In both of these examples, the result of the sum was a whole number. Now, we are ready to try
some examples where the sum is a negative integer. In this case you will be trying to show that the
sum is the opposite of some positive number. For example to prove that 3 + ( 7) = 4 you must
show that 3 + ( 7) is the opposite of 4: What does it mean to be the opposite of 4? The opposite
of 4 is the number that when added to 4 gives 0: So, you must show that 4 + (3 + ( 7)) = 0:
Example 99 Prove 3 + ( 7) = 4:
4+ 3+
7
= (4 + 3) +
= 7+
7 by the associative property of addition of integers
7 by whole number arithmetic
= 0 by de…nition of opposites.
Therefore, 4 + (3 + ( 7)) = 0:
So, by the de…nition of opposites, 3 + ( 7) = 4:
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72
Example 100 Prove ( 1) + ( 5) = 6:
6+
1 +
5
= (5 + 1) +
= 5+ 1+
1 +
1
+
5
by whole number arithmetic
5 by associative property of addition of integers
= 5 + 0 + 5 by de…nition of opposites
= 5 + 5 by additive identity property of 0
= 0 by de…nition of opposites.
Therefore, 6 + (( 1) + ( 5)) = 0:
So, by the de…nition of opposites, ( 1) + ( 5) = 6:
Absolute Value
De…nition
The absolute value of a number is de…ned by a piecewise de…ned function. This is because the
formula used for computing the absolute value of a number depends on whether the number is
positive, zero or negative. If a number is positive or zero, then the absolute value of that number
is simply equal to that number. However, if the number is negative, then the absolute value of the
number is equal to its opposite.
De…nition 101 For any integer x; the absolute value of x; denoted jxj is determined by the following piecewise de…ned function
jxj =
x if x 0
x if x < 0
Notice that the absolute value of a number will never be negative. If the number x is not
negative, then jxj = x is not negative. On the other hand, if x is negative, then jxj is equal to the
opposite of x which will be positive (since the opposite of a negative number is positive).
Example 102 Compute j5j :
Since 5 0; then j5j = 5:
Example 103 Compute j 3j :
Since 3 < 0; then j 3j = ( 3) = 3:
Example 104 Compute j4 ( 2)j :
To determine this using the de…nition we …rst have to …nd 4 ( 2) : Since 4
have j4 ( 2)j = j6j : So, since 6 0; we have j4 ( 2)j = j6j = 6:
( 2) = 6; we
Geometric Interpretation
The de…nition for jxj given in the previous section is useful, but it does not give us much intuition
for jxj : The geometric interpretation is much more helpful in that respect.
Geometric Interpretation: The geometric/numberline interpretation of jxj is that jxj is the
distance from x to the origin (0) on the numberline.
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73
Example 105 Use the geometric interpretation to compute j5j :
Since 5 is 5 units from 0 on the numberline, j5j = 5:
Example 106 Use the geometric interpretation to compute j 3j :
Since
3 is 3 units from 0 on the numberline, j 3j = 3:
It is clear from the geometric interpretation of absolute value that the absolute value of a number
will never be negative. The geometric interpretation says that the absolute value of a number is a
distance; we know that distance cannot be negative, thus, the absolute value of a number cannot
be negative.
There is a further geometric interpretation of absolute value when concerned with the absolute
value of a di¤erence; e.g. jx yj :
Geometric Interpretation of jx yj : The geometric/numberline interpretation of jx
jx yj is the distance between x and y on the numberline.
yj is that
Notice that when y = 0 this says that jx 0j is the distance between x and 0 on the numberline.
That is, jxj is the distance between x and 0 on the numberline. This is simply the …rst geometric
interpretation given in this section.
Example 107 Use the geometric interpretation to compute j4
Since the distance between 4 and
( 2)j :
2 on the numberline is 6 units, j4
( 2)j = 6:
Having this geometric interpretation of absolute value is invaluable to students when they begin
studying equations and inequalities involving absolute values in algebra.
Example 108 Solve the following inequality for x : jx 2j 3:
Using the geometric interpretation from above, we can interpret this inequality as
the distance between x and 2 is less than or equal to 3 units.
Let’s look at a numberline and …gure out what numbers are no more than 3 units away from 2.
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74
To do this, I started at 2 and moved left and right 3 units each. Now those numbers in the
shaded region are no more than 3 units away from 2. So, they are solutions to my inequality.
Thus, the solution to this inequality is that x is a solution if 1 x 5:
Inequalities
De…nitions
We start with less than, <, and less than or equal to,
:
De…nition 109 For any integers a and b; we say that a is less than b; written a < b; if there is
a natural number c such that a + c = b:
Note that this is saying that a < b if you have to add something positive to a to get b:
De…nition 110 For any integers a and b; we say that a is less than or equal to b; written a
if there is a whole number c such that a + c = b:
b;
Note that by allowing c to be a whole number here we are allowing for c to be 0 in which case
a + c = b would mean a = b:
Example 111 True or false: 7 < 23:
True since 7 + 16 = 23 and 16 2 N:
Example 112 True or false: 23 < 23:
False since 23 + 0 = 23 and 0 62 N:
Example 113 True or false: 4 0:
True since 4 + 4 = 0 and 4 2 W:
Now we are ready for greater than, >, and greater than or equal to,
terms of the two we have already de…ned.
: We de…ne these in
De…nition 114 For any integers x and y; we say that x is greater than y; written x > y; if
y < x:
Note that this means that x > y if there is a natural number c such that y + c = x:
De…nition 115 For any integers x and y; we say that x is greater than or equal to y; written
x y; if y x:
Note that this means that x
y if there is a whole number c such that y + c = x:
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75
Example 116 True or false: 19
13:
This is false since 13
19 is false
since 13 + 6 = 19 and 6 62 W:
Example 117 True or false: 12 > 14:
This is true since 14 < 12 is true
since 14 + 26 = 12 and 26 2 N:
Geometric Interpretation
A greater intuition for inequalities can be built by using the geometric interpretation of each of the
four types of inequalities.
a < b if a is to the left of b on the numberline.
a
b if a is to the left of or at b on the numberline.
c > d means, by de…nition, that d < c which means d is to the left of c on the numberline so
c > d means that c is to the right of d on the numberline.
c
d means, by de…nition, that d
c which means that d is to the left of or at c on the
numberline so c d means that c is to the right of or at d on the numberline.
Example 118 Use the geometric interpretation to determine each of the following
True or false: 7 < 23:
This is true since 7 is to the left of 23 on the numberline.
True or false: 23 < 23:
This is false since 23 is NOT to the left of 23 on the numberline.
True or false:
19
This is false since
13:
19 is NOT to the right of or at
13 on the numberline.
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76
Multiplication of Integers
To be able to multiply integers we must determine the “rules of sign” for multiplication. That is,
1. positive
positive = positive,
2. positive
negative = negative,
3. negative
positive = negative, and
4. negative
negative = positive.
positive
positive = positive
We already know this fact since this is simply saying that a natural number multiplied by a natural
number is a natural number.
positive
negative = negative
To de…ne the product of a positive integer and a negative integer, we simply take a natural extension
of the de…nition of multiplication of natural numbers. Recall that, by de…nition, 4 3 = 3+3+3+3:
So, it is natural to let 4 ( 3) = ( 3) + ( 3) + ( 3) + ( 3) : So, 4 ( 3) = 12: In fact we get
from this that a ( b) = (ab) :
negative
positive = negative
Now we are looking at a product such as ( 4) 3: We cannot simply extend the de…nition we
used for natural number multiplication here because that de…nition would tell us that, to compute
( 4) 3, you simply add 3 to itself 4 times, but that doesn’t make any sense.
One option we have here is to assume that the commutative property of multiplication of integers
holds (which it does), in which case a negative positive becomes a positive negative which we
have already dealt with.
An option I like to use to convince a child that a negative
positive should be negative is
patterns. We start with the following whole number arithmetic.
4
3
2
1
0
3
3
3
3
3
= 12
= 9
= 6
= 3
= 0
There are some clear patterns here. First, the numbers in the …rst column decrease by one
with each successive entry. Secondly, the numbers in the last column decrease by three with each
successive entry. This is because you are adding fewer threes with each successive line (e.g. the
…rst line is 4 3 which is 3 + 3 + 3 + 3 while the second line is 3 3 which is 3 + 3 + 3): If you
continue with these patterns for a bit you get
1
2
3
4
5
3
3
3
3
3
=
=
=
=
=
3
6
9
12
15
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77
So, we see that, for example, 4 3 = 12: More generally, we see that a negative
is a negative. From this we get ( a) b = (ab) :
negative
positive
negative = positive
Suppose we want to show, for example, that 4
3 = 12: We can continue with the pattern idea
we used above to get this. Remember, we already know that positive negative = negative, so we
can …ll in the following.
3
3 =
9
2
3 =
6
1
3 =
3
0
3 = 0
Notice the number in the …rst column of this chart are decreasing by one as you move down
the chart while the numbers in the …nal column are INCREASING by three as you move down the
chart. So, if we continue the chart, we expect to see the pattern continue to give us the following.
1
2
3
4
3
3
3
3
= 3
= 6
= 9
= 12
So, we see that, for example, ( 4) ( 3) = 12: More generally, we see that a negative
is positive. From this we get ( a) ( b) = ab:
negative
Division of Integers
Rules of Sign
Since division is de…ned in terms of multiplication, we can determine the rules of sign for division
by using the rules of sign for multiplication. The rules of sign for division are as follows:
1. positive
positive = positive,
2. positive
negative = negative,
3. negative
positive = negative, and
4. negative
negative = positive.
To see how these follow from the rules of sign for multiplication, let’s look at an example.
Example 119 Use the rules of sign for multiplication to help in proving that positive
negative
= negative.
Suppose a is a positive integer and b is a negative integer and that a b = c: We need to prove
that c is negative.
By de…nition of division, since a b = c; we have a = b c:
We know that a is positive and b is negative. So, here we have positive (a) = negative (b)
times something??? (c) : Looking at our rules of sign for multiplication we see that for a negative
something to be positive, that something must be negative (i.e. negative negative = positive).
So, c must be negative. That is positive negative = negative.
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78
Computation Without Fractions
If we understand the de…nition of division well enough, we can make some computations that we
would normally expect to use fractions or decimals to compute without ever leaving the integers
(i.e. without using fractions or decimals).
Example 120 Without using fractions or decimals, compute (3
Let 3 5 = c: Then, by de…nition of division 3 = 5c:
So,
(3
5) 10:
5) 10 = (c) 10 by substitution
= 10c by the commutative property of multiplication of integers
= (2 5) c by integer arithmetic
= 2 (5c) by the associative property of multiplication of integers
= 2 (3) by substitution (remember 3 = 5c above)
= 6:
So, (3
5) 10 = 6:
Example 121 Without using fractions or decimals, compute (6 10) 15:
By integer arithmetic, we know 6 10 = 60: So, we want to compute 60 15:
If 60 15 = c; then 60 = 15c: By integer arithmetic, we know 60 = 15 4: So, c = 4: Thus,
60 15 = 4:
So, (6 10) 15 = 4:
Chapter 4 Exercises
1. Given that x is a whole number. What is the de…nition of x:
2. Are there any whole numbers x such that x = x? Explain (your explanation should probably
refer to the de…nition of x):
3. Use colored counters to …nd the following sums. Make a sketch of your work. You may use
as many counters as your wish.
(a) 1 + ( 5)
(b) 6 + ( 4)
(c) 3 + ( 3)
(d) ( 5) + ( 2)
4. Use colored counters and the take-away model of subtraction to compute each of the following.
(For this problem do not change these subtraction problems into addition problems–I want
you to use the take-away model.)
(a) 3
2
(b) 3
5
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79
(c) ( 2)
3
(d) ( 3)
( 2)
5. Compute each of the following: (you should be able to do this without a calculator)
(a)
2+3 5
(b) 10
3 7
(c) 10
3
(d) 10
(3
1
4 ( 2) + 3
12
12)
2
(e) ( 3)
(f)
32
(g)
52 + 3 ( 2)
(h)
23
2
5
(i) ( 2)
(j)
24
6. If x is an integer and x 6= 0; determine whether each of the following is always positive, always
negative or sometimes positive/sometimes negative.
(a)
x2
(b) x2
2
(c) ( x)
(d)
x3
3
(e) ( x)
(f)
x4
4
(g) ( x)
(h) x4
(i) x
(j)
x
7. Which of the expressions in problem 6 are equal to each other for all values of x:
8. Using only the properties of the whole numbers and the de…nition of opposites (i.e. a+ a = 0)
prove each of the following. Show your work one step at a time–use only one property at a
time.
(a) 4 + ( 3) = 1
(b) ( 7) + 13 = 6
(c) ( 2) + ( 3) = 5
(d) 4 + ( 6) = 2
9. Solve each of the following subtraction problems by rewriting each as an addition problem.
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80
(a)
8
7
(b) 12
6
(c)
11
15
(d)
9
15
10. Give the algebraic de…nition of the absolute value, jxj :
11. Use the algebraic de…nition of absolute value to evaluate each of the following. Show
your work (make sure it’s clear that you’re using the de…nition)
(a) j17j
(b) j 28j
(c) j 28
(d) j 5
7j
18j
12. Use the numberline to determine each of the following. Justify each answer (draw a picture
if applicable).
(a) j17j
(b) j 28j
(c) j 28
(d) j 5
7j
18j
13. Solve each of the following equations involving absolute values.
(a) jxj = 1
(b) jxj = 3
(c) jxj + 2 = 5
(d) jx + 2j = 5
14. Use the de…nition of <;
Justify each answer.
; >;
to determine whether each of the following is true or false.
(a) 7 < 23
(b)
19 > 23
(c)
35
45
15. Use the number line to determine whether each of the following is true or false. Justify
each answer.
(a)
15
13
(b) 23 < 23
(c) 12 > 14
16. Compute each of the following pairs of expressions (two computations per problem):
The Integers
81
(a) j7 + 2j and j7j + j2j
(b) j( 8) + 5j and j 8j + j5j
(c) j7 + ( 6)j and j7j + j 6j
(d) j( 9) + ( 5)j and j 9j + j 5j
(e) j6 + 0j and j6j + j0j
17. Based on the results from problem 16, answer each of the following:
(a) Is it ever true that ja + bj = jaj + jbj : If yes, give an example.
(b) Is it always true that ja + bj = jaj + jbj : If no, give a counterexample. That is, provide
an a and a b such that ja + bj =
6 jaj + jbj :
(c) Which of the following symbols (<, ; >,
make a statement that is always true.
ja + bj
; =) can be placed in the blank below to
jaj + jbj
18. We saw in class that the de…nition of opposites can be restated as
x + y = 0 if and only if y = x
(i.e. if and only if y is the opposite of x): You may …nd this version of the de…nition helpful
in answering the following question.
Suppose a and b are integers.
(a) If c is equal to the opposite of a + b (i.e. c = (a + b)) what can we say about c: That
is, what equation must c satisfy (based on the de…nition above!!!!).
(b) Show that a + b = (a + b) by showing that a + b satis…es the equation that you
gave for c above.
19. When integers are …rst introduced in the (elementary school) classroom it helps with the
students understanding if you can provide some examples of places in everyday life where
negative numbers show up. A common …rst example is a thermometer. This is an especially
good example in places where the temperature regularly dips below zero, so that the students
are accustomed to hearing about temperatures of 15 below zero. The thermometer also
works well as a model because it is essentially a numberline. You can phrase addition and
subtraction problems in terms of the temperature. The addition fact 15 + 10 can be thought
of as, if it’s 15 below zero now and the temperature rises 10 ; what will the new temperature
be? Similarly, the subtraction fact 15 10 can be thought of as, if it’s 15 below zero now
and the temperature falls 10 ; what will the new temperature be. Come up with another
example of a real-world situation where negative numbers are used regularly (preferably one
that kids would be familiar with). Discuss how thinking of the integers in that setting could
help someone to understand the integers better.
20. Use the de…nition of division to …nd each quotient, if possible. If a quotient is not de…ned,
explain why.
(a)
40
8
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82
(b) 143
(c)
( 11)
5
(d) 0
0
0
21. Which rule of signs for multiplication gives us
(a) negative
positive = negative?
(b) positive
positive = positive?
(c) negative
negative = positive?
22. Evaluate each of the following using the properties we’ve learned in this class and the de…nition
of division, if possible: (remember, don’t use fractions!!!)
(a) ( 10
(b) (a
2) ( 2)
b) b
(c) ( 10 5)
(d) (ab)
(e) ( 8
(f)
8
5
b
2) ( 8)
( 8 + 8)
(g) ( 8 + 8)
8
(h) ( 6 + 6)
( 2 + 2)
(i) ( 36
12)
3
(j) ( 23
7)
4
(k) j 24j
(3
15)
23. We saw in class that
a
b=a+
b :
Also, in algebra we learn to use the distributive property to compute (x + y)2 = (x + y) (x + y) =
x2 + 2xy + y 2 ; so
(x + y)2 = x2 + 2xy + y 2 :
(a) Use the two facts above to …nd a formula for (a
b)2 :
(b) Use the formula you found in part (a) to compute each of the following:
i. 982 (Hint: Write 98 = 100
ii. 992
iii. 9972
2)
Index
additive identity, 65
additive inverse, 65
arithmetic sequence, 11
complement A , 14
complex numbers, 11
composite, 52
divides, 51
division algorithm, 43
divisor, 51
element, 7
emptyset, 7
equal sets, 7
equivalent sets, 22
factor of, 51
…nite set, 24
FTA, 56
prime power representation, 57
rational numbers, 11
real numbers, 11
roster notation, 8
rules of sign, 76
set, 7
set di¤erence (A B), 14
set-builder notation, 9
subtraction as addition lemma, 70
union (A [ B), 13
unit, 52
universe, 14
whole numbers, 10
without redundancy, 8
greater than, 74
greater than or equal to, 74
greatest common divisor
gcd, 58
in…nite set, 24
integers, 11
intersection (A \ B), 13
least common multiple
lcm, 58
less than, 74
less than or equal to, 74
multiple of, 51
natural numbers, 10
one-to-one correspondence, 22
opposite, 65
prime, 52
83