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Section 2.8 The Reciprocal of a Real Number
Objective: To simplify expressions involving reciprocals
Definition: Two numbers whose product is 1 are called reciprocals, or
multiplicative inverses, of each other.
Examples:
1. 1 and
2.
1
1
are reciprocals because 5   1 .
5
5
4
5
4 5
and
are reciprocals because   1 .
5
4
5 4
3. -1.25 and -0.8 are reciprocals because (-1.25)(0.8) = 1.
4. 1 is its on reciprocal because 1  1  1 .
5. -1 is its own reciprocal because (-1)(-1) = 1.
6. 0 has no reciprocal because 0 times any number is 0, not 1.
Property of Reciprocals
For every nonzero real number a, there is a unique real number
a
1
1
 1 and  a  1
a
a
Also: The reciprocal of  a is 
1
.
a
Examples:
Simplify:
1.
1 1

4 7
4.
1
(42m  3v)
3
2. 4 y 
1
4
 1
3. ( 6ab)  
 3
1
such that:
a
Section 2.9 Dividing Real Numbers
Objective: To divide real numbers and to simplify expressions involving
quotients.
1
1
. 8  2  4 and 8   4 .
2
2
1
1
**Dividing by 5 is the same as multiplying by . 15  5  3 and 15   3.
5
5
**Dividing by 2 is the same as multiplying by
Definition: Dividing by a number means multiplying by its reciprocal. That
is, for any real numbers x and y with y  0 ,
x y  x
1
y
or
x
1
 x
y
y
Rules for Division:
**Negative divided by a negative is positive.
**Positive divided by a negative is negative.
**Negative divided by a positive is negative.
Examples:
27
 10 
 3
1. (9)     = (9)   =
10
 3
 10 
2.
3
 1
= (3)     = (3)( 3) = 9
1
 3

3
Questions:
1. Why can you never divide by zero?
Dividing by 0 would mean multiplying by the reciprocal of 0. But
we learned earlier that 0 has no reciprocal.
2. Can you divide by any number other than zero?
Yes!!
2.7 Warm-up
Simplify:
1
= a
3
1.
(3a )
2.
1
(10 z  12) = 5z + 6
2
3.
1
1
1

=
 2  12
24
 1
4. 8     = -48
 6
5.
 3
n
= -n
3
Section 2.7 Problem Solving: Consecutive Integers
Objective: To write equations to represent relationships among integers.
Definition: When you count by ones from any number in the set of integers,
{… ,-3, -2, -1, 0, 1, 2, 3} you obtain consecutive integers.
Example:
1. An integer is represented by n.
a). Write the next three integers in natural order after n.
b). Write the integer the immediately precedes n.
c). Write an equation that states that the sum of four
consecutive integers, starting with n, is 66.
Solutions:
a). n + 1, n + 2, n + 3
b). n – 1
c). n + (n + 1) + (n + 2) + (n + 3) = 66
Even integer – an integer that is the product of 2 and any integer.
Odd integer – an integer that is not even.
Consecutive even integers – if you count by twos beginning with any even
integer.
Consecutive odd integers –if you count by twos beginning with any odd
integer.
Examples:
Write an equation to represent the given relationship among integers.
1. The sum of three consecutive integers is 75.
2. The product of two consecutive even integers is 168.
3. The sum of three consecutive odd integers is 40 more than the smallest.
What are the integers?