Adding and Subtracting Unlike
Fractions
3.6
To add and subtract fractions with unlike
denominators, first find a common denominator
using one of these methods:
Method 1
Find a common denominator by
multiplying the two denominators.
Method 2
Find the least common
denominator (LCD) of the
denominators. (Cake Method)
Steps
• Find LCD or CD
• Write equivalent fractions: Multiply the
fraction by a value of 1 in order to get the LCD
or CD
• Add or subtract the numerators
• Reduce
Write in Simplest form:
5
1

9
6
• LCD:
18
• Rename Fractions:
5 2 10
 
9 2 18
1 3 3
 
6 3 18
• Add the numerators:
10 3

18 18
10  3

18
7

18
Write the answer in simplest form.


5 3  2 4
5 4
5 4
15 + 8
20 20
23  1 3
20
20
Find a common denominator: 4(5) = 20.
Multiply by fractions equal to 1.
Rewrite with common denominators.
Add numerators.
Keep the denominator.
Rewrite as a mixed number.
Write in Simplest Form:
5
1

5   2 
6
4

2 35  9  3
   
2 6  4 3
70 27

12 12
97
1
8
12
12
Find the perimeter of the figure below.
2
1 in.
3
1
1 in.
4
1
2 in.
2
First, work with
the fractions.
Then, work with
the whole
numbers.
Last, simplify.
+



2
1
3
1
1
4
1
2
2
4 8
1
12
4
3  3
1
12
3
6  6
2
12
6
17
4
12
5
5
41  5
12
12
2
Vickie has 1 3
cups of shredded cheese to put in her
enchilada casserole and ¾ cup of cheese for the top. If
she has 3 cups of cheese in all, how much cheese will
3
 2
3  1


she have left?
4
 3
3
5
3


4
 3
5
3
3
 4
3




3
3
4
 4
9 
 20
3


12 
 12
29
3
12
12
29 

3


12
12 

36
 29 


12
 12 
7
12
Homework:
• Page 129 (14-46) even