Download Adding and Subtracting Fractions

3
8
Wednesday, October 20 (Blue)
Thursday, October 21 (Gold)
Objective Students add and subtract fractions and mixed numbers
•
•
Fill in planner
•
Today’s Assignment (Practice 5-3)
Bell Work
•
Compare the fractions using <,>, or =
3
2
 
4
3
2 1

3 4
5 3

10 6
7 9

8 10
Adding Fractions with common
denominators
3 4
7


8
8 8
Just add the numerators!
Adding Fractions with different
denominators
Problem:
You can’t add fractions with different denominators.
Solution:
Turn fractions into equivalent fractions with a
common denominator,
preferably the LCD!
Finding the
Lowest Common Denominator
• The lowest common multiple of two numbers is
the lowest number in BOTH lists of multiples
Example 1
1 1

2 3
Multiples of 2 are 2, 4, 6, 8, 10……
Multiples of 3 are 3, 6, 9, 12, ………
What is the lowest common
multiple?
Finding the
Lowest Common Denominator
• The lowest common multiple of two numbers is
the lowest number they will BOTH divide into
1 1

2 3
2 divides into 2, 4, 6, 8…..
3 divides into 3, 6, 9….
What is the lowest number 2 and 3
both divide into?
1 1

2 3
You can’t add fractions with
different denominators
+
The Lowest Common Multiple of 2 and 3 is 6 so turn all fractions into sixths
1 3 3
 
2 3 6
Special form of 1
1 2 2
 
3 2 6
3 2 5
 
6 6 6
Example 2
1
2

2
5
Lowest common denominator is 10 so make all fractions tenths
5 4 9
 
10 10 10
Example 3:
1
1

3
4
Turn both fractions into twelfths
4 3
7
 
12 12 12
Variable Expressions
Example 4
1 5x

8 6
1 3 5x  4

83 6 4
3  20 x
24
Variable Expressions
Example 5
24
3m
2
4

3m 3m
2

3m
Variable Expressions
Example 6
y 5 y 2

25 5 2
5y 2y

10 10
y y

2 5
7y
10
Adding Mixed Numbers
Find the sum.
1
+
3
4
9
1. Rewrite the fraction using
LCD if denominators are
different.
1
2. Add or subtract the fractions,
4
9
5
9
then the whole numbers.
3. Simplify if possible.
In this problem, the denominators are the
same, so begin with step 2.
Let’s Do More Examples!
Example 8
1
+
3
4
Example 9
4
4
9
1
3
7
9
X3
=
X3
3
9
+
5
7
8
X2
1
4
=
X2
8
9
9
10
2
8
1
8
= 1
1
8
Add the whole numbers.