Multiply each number by x1, x2, x3 (etc) to determine MULTIPLES.

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Stop when you find the FIRST multiple they have in common.​

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Can use for creating common denominators.

​Least common multiple

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You can find common denominators by finding the LCM of the denominators, or by multiplying by the opposite denominator.

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​NOTE: Multiplying by the opposite denominator makes simplifying your fractions more difficult.

finding a common denominator

We cannot add or subtract fractions with unlike denominators.

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You have to create a common denominator by finding the LCM of BOTH denominators.​

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• See LCM slide.

• See Finding Common Denominators slide​.

adding and subtracting fractions

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Find the LCD of the denominators.

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We see that 35 is the LCD of 5 and 7.​

step one

​Multiply your first fraction by a fraction equivalent to ONE.

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We multiply by 7/7 because that gives us our LCD of 35.​

step two

Multiply your second fraction by a fraction equivalent to ONE.

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We multiply by 5/5 because that gives us our LCD of 35.​

step three

Add/subtract ONLY the numerators, and leave the denominator ALONE!

step four

List out every number that can be multiplied to get the multiple (break apart the multiple into its teeny-tiny factor pieces).

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Find the BIGGEST factor the numbers have in common.​

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Use for simplifying fractions.

greatest common factor

To simplify your final fraction sum/difference, find the GCF of both the numerator AND the denominator.

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Divide the numerator and the denominator by the GCF.​

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• ​See GCF slide.

​simplifying fractions

Complete 5 divided by 3.

The whole number in the quotient becomes your whole number in the mixed fraction.

The remainder becomes your new numerator.

THE DENOMINATOR REMAINS THE SAME.​

improper to mixed

Multiply the denominator by the whole number, then add the numerator. THIS IS YOUR NEW NUMERATOR.

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LEAVE THE DENOMINATOR THE SAME.​

​​Mixed to improper

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​Practice: Adding and Subtracting Fractions

​show what you know

Complete the following practice questions on adding and subtracting fractions using the strategies we've just reviewed.

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You may use a white board, or complete your work inside your MSG.​

You can go back and look at the previous slides if you're having trouble.