1. Mrs. Brenda Windsor
2008

2. 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
=

4. +
2
9
=
9
5
9
16
+
4
16
=

5. 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?

6. +
2
9
=
9
4
+
3
12
=
12
6

7. +
2
15
=
15
7
+
1
16
=
16
1

8. 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?

10. +
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)

11. 3: 3, 6, 9, 12, 15, 18
6: 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)

12. Adding Fractions with Unlike
Denominators
3
7
+
3
4
=
Multiples of 7:
Multiples of 4:
7, 14, 21, 28
4, 8, 12, 16, 20, 24, 28

13. 3
7
12
28
=
3
4 =
21
28
X 4
X 4
X 7
X 7
3 3
7 4
+
12 21
28 28
+
START
READY
TO ADD

14. Common Multiples for these numbers:
6:
12:
What is the LCM?

15. Common Multiples for these numbers:
8:
12:
What is the LCM?

16. Common Multiples for these numbers:
9:
12:
What is the LCM?

17. Common Multiples for these numbers:
4:
12:
What is the LCM?

18. Common Multiples for these numbers:
3:
5:
What is the LCM?

19. Common Multiples for these numbers:
7:
16:
What is the LCM?

20. Common Multiples for these numbers:
6:
5:
What is the LCM?

21. +
2
3
=
9
4
What is the LCM that you need for a
common denominator?

22. +
4
9
=
12
5
What is the LCM that you need for a
common denominator?

23. +
4
7
=
9
5
What is the LCM that you need for a
common denominator?

24. +
2
3
=
6
5
What is the LCM that you need for a
common denominator?

25. +
4
9
=
2
1
What is the LCM that you need for a
common denominator?

26. 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?