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## Susan Bryan

on 24 September 2013

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#### Transcript of Adding and Subtracting Fractions

Conclusion
Adding and subtracting fractions is easy if you remember these simple rules.
Find a common denominator
Multiply both the numerator and denominator
Denominators
tell us how many pieces a whole item was broken into.
Numberators

tell us how many pieces we have.

1/4 means we have 1 out of 4 pieces that make a whole item.
2/3 means we have 2 out of 3 pieces that make a whole item.

We can add and subtract pieces that are the same size in order to make wholes and parts of wholes.
Once you know the common denominator, you must multiply the entire fraction by the same multiplier.
1/2 + 3/7 has a common denominator of 2 * 7 or 14

1/4 + 1/6 has a common deominotor of 12 since both 4 and 6 divide evenly into 12.
Making Denominators the Same
There are several ways to make the denominators the same size.
Multiply the denominators
Find the lowest common multiple

1. What is the common denominator of 2/5 and 3/4?
2. 7/8 and 1/2?
3. 1/6 and 1/9?
Subtracting Fractions
Fractions are subtracted in the same way.
Find a common denominator
Multiply the numerator and denominator by the same number
Subtract
3/4 - 1/3
Common denominator is 4 x 3 = 12
3/4 x 3/3 = 9/12
1/3 x 4/4 = 4/12

9/12 - 4/12 = 5/12

So 3/4 - 1/3 = 5/12
5/8 - 1/2
Common denominator is 8 because 2 divides into 8.
5/8 x 1/1 = 5/8
1/2 x 4/4 = 4/8

5/8 - 4/8 = 1/8

So 5/8 - 1/2 = 1/8
4 pieces in 1 whole
We have 2 pieces
2 pieces out of 4 pieces is written as 2/4.
2/4 can reduce to 1/2.
Questions
10. What does the numerator of a fraction tell you?
11. What does the denominator of a fraction tell you?
12. What is the first thing you must do in order to add or subtract fractions?
13. When you change the denominator, what happens to the numerator?

Be sure you worked the nine problems in the presentation!

Once You Know the Denominator
1/2 + 3/5 has a common denominator of 2 x 5 or 10. To change 2 to 10, it must be multiplied by 5. However the numerator must be multiplied by 5 also. 1/2 becomes
1/2 x 5/5 = 5/10.

In the same way 3/5 becomes 3/5 x 2/2 = 6/10.

Now we can add them. 5/10 + 6/10 = 11/10.
Therefore 1/2 + 3/5 = 11/10.