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Mrs. Brenda Windsor 2008

Adding fractions

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Mrs. Brenda Windsor

2008

Adding Fractions with Common Denominators

1. Add the Numerators.

2. Keep the Denominators the same.

3 5+

11 11=

8

11

3 5+

11 11=

+2

9=

9

5

9

16+

4

16=

Reduce or simplify if necessary…

2

15+

3

15=

5

15

5

15÷

5

5=

1

3

What does it mean to

reduce or simplify?

+2

9=

9

4

+3

12=

12

6

+2

15=

15

7

+1

16=

16

1

Adding Fractions with Unlike Denominators

1. Find a common denominator using LCM.

2. Multiply numerator and denominator by LCM.

3. Add the converted fractions.

What is an LCM?

+2

3

=

6

5

We need to create fractions with like denominators in order to add these fractions. This is where we need LCM (Least Common Multiples)

3: 3, 6, 9, 12, 15, 186: 6, 12, 18

•The first common multiple is the Least Common Multiple

• This will become the new denominators in order to create like denominators to add fractions.

•List the multiples for each of the factors (denominators)

Adding Fractions with Unlike Denominators

3

7+ 3

4=

Multiples of 7:

Multiples of 4:

7, 14, 21, 284, 8, 12, 16, 20, 24, 28

3

71228=

3

4 =

21

28

X 4

X 4

X 7

X 7

3 3

7 4 +

12 21

28 28 +

START

READY TO ADD

Common Multiples for these numbers:

6:

12:

What is the LCM?

Common Multiples for these numbers:

8:

12:

What is the LCM?

Common Multiples for these numbers:

9:

12:

What is the LCM?

Common Multiples for these numbers:

4:

12:

What is the LCM?

Common Multiples for these numbers:

3:

5:

What is the LCM?

Common Multiples for these numbers:

7:

16:

What is the LCM?

Common Multiples for these numbers:

6:

5:

What is the LCM?

+2

3=

9

4

What is the LCM that you need for a common denominator?

+4

9=

12

5

What is the LCM that you need for a common denominator?

+4

7=

9

5

What is the LCM that you need for a common denominator?

+2

3=

6

5

What is the LCM that you need for a common denominator?

+4

9=

2

1

What is the LCM that you need for a common denominator?

REVIEW

• In order to add fractions, what must you have?

• Explain a simplified fraction

• What is an LCM?

• What are LCMs used for?

• Why is it important to be able to find an LCM?