Transcript Multiplying and Dividing Fractions

Multiplying and Dividing
Fractions
7th Grade Math
August, 2013
A Quick Review:
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KEY POINT when multiplying or dividing
fractions:
Change a mixed number into
an improper fraction,
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Still simplify your answer.
More on Multiplying Fractions:
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“of” means multiply!
Example:
 There are 8 cars in Michael’s toy collection. 1/2 of
the cars are red. How many red cars does Michael
have?
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This problem is asking “What is 1/2 of 8?”
A way to answer it is to put a multiplication sign in place of “of.” You
then get 1/2 x 8 or 8 x ½ (remember that multiplication is
commutative).
Multiplying Fractions:
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Multiplying fractions is easier than adding or
subtraction because you don’t need to find
common denominators. YAY!!!!!!
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Just multiply straight across.
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Multiply numerators
Then, multiply denominators
A Few Examples:
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Example #1: 2/3 X 4/5
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Answer: 8/15
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Example #2: 9/2 X 3/7
 Answer: 27/14=1 13/14
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Example #3: 2 1/6 X 3/2
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Answer: 39/12=3 3/12=3 ¼
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Example #4: 5 X 2/7
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Answer: 10/7=1 3/7
Example #5:
5 1/3 • 9 ½
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Answer: 50 2/3
Make Life Easier!!
Cross Reduce
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When multiplying, you can simplify your
factors by “cross reducing”.
Examples:
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6/35 • 5/24
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2/15 • 3/18
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1/8 (4/5)
What in the World is a
“Reciprocal”?
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Reciprocal:
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When two fractions are multiplied and their
product is 1.
AKA “inverting” or “flipping” a number
2 3
Examples:
 1
3 2
 The reciprocal of ½ is _______.
 The reciprocal of 1 ¾ is _______.
 The reciprocal of 8 is ________.
Algorithm: Dividing Fractions
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Remember these three words:
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KEEP, CHANGE, RECIPROCAL
KEEP First fraction
CHANGE Division to multiplication
RECIPROCAL of last fraction
Multiply and Simplify
Some Examples:
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5
Example #1: 3

4
6
8
2
Example #2:

3 4
Example #3: 5  1
3
1
2
Example #4: 6 4  2 3
Don’t forget
to cross
reduce if
possible
ONLY when
multiplying!
“Card and Domino Multiplication/Division”
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Each partner draws one (1) card and one (1)
domino
 Card = whole number
 Domino = fraction
Multiply your two numbers (individually)
Divide your two numbers (individually)
Compare your answers. If you both have the
same answer, give your team a point for each
problem. Highest points (with work shown) wins
candy!