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Adding and Subtracting Fractions

Adding and Subtracting Fractions with different denominators

Rebecca Chee

on 15 November 2013

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Transcript of Adding and Subtracting Fractions

Finding Common Denominators in order to add and subtract fractions!
Adding and Subtracting Fractions
1. What is a denominator?
2. What does putting a fraction in "simplest terms" mean?
3. What is a Greatest Common Factor?
4. What do you think the "Lowest Common Denominator" is?

5. What is 1/4 + 2/4? (Draw a Picture!)

6. What is 1/4 + 1/2? (Draw a Picture)
Just to Get You Thinking!
Adding and Subtraction Fractions with Common Denominators
Adding fractions with the same denominator is easy - we just add the numerators together!
Imagine you have a pizza with 8 slices, and you eat 3 slices of pizza for lunch, and then you eat 4 slices of pizza for dinner. What fraction of the pizza is left over for tomorrow's breakfast?
Adding and Subtracting Fractions with Different Denominators
Imagine that you needed to buy 2 pizzas, and one came in 6 slices, and one came in 8 slices. You ate 4/6 of the first pizza, and your friend ate 3/8 of the second pizza. What fraction of the two pizzas is left?
Obviously, this question is a little harder, so...
let's compare these problems!
What is different about them?

the first one has common denominators!
How would we "force" the second problem to have common denominators?
Let's get as far as we can...
Method 1: Listing Multiples!
If the denominators are small numbers, we should list the multiples of both denominators to find the
lowest common multiple (LCM)
, then we multiply the fractions to change them to equivalent fractions with the

lowest common denominator (LCD)
, and then add or subtract like normal.
Method 2: If All Else Fails...
If all else fails... multiply each fraction (top and bottom) by the other fractions' denominators!
Remember! We always put our answers in lowest form!
Remember! We always put our answers in lowest form!
Least Common Multiple:
- the smallest same multiple between two or more numbers

Least Common Denominator:
the smallest possible same denominator between two or more fractions
Key Words
Learning Objective:
to be able to add and subtract fractions with different denominators
Success Criteria
I can add and subtract fractions with different denominators
Adding and Subtracting Fractions
Full transcript