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## Multiplying and Dividing Fractions

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### Multiplying and Dividing Fractions 7th Grade Math August, 2012 Lesson Overview: This lesson will serve as a REVIEW of multiplying and dividing fractions. – PowerPoint PPT presentation

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Title: Multiplying and Dividing Fractions

1
Multiplying and Dividing Fractions
• August, 2012

2
Lesson Overview
• This lesson will serve as a REVIEW of multiplying
and dividing fractions.
• If you struggle with this concept, please pay
close attention and lets master this stuff!!!

3
MULTIPLYING FRACTIONS ANDMIXED NUMBERS!
4
Discovery Education Videos
Multiply Fractions
Multiply Mixed Numbers
5
A Quick Review
• KEY POINT when multiplying or dividing fractions
• Change a mixed number into
• an improper fraction,

6
More on Multiplying Fractions
• The word of in a problem usually means
multiply!
• Here is an example
• There are 8 cars in Michaels toy collection.
1/2 of the cars are red. How many red cars does
Michael have?
• 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).

7
Multiplication Continued
• What operation will I use for 2/3 of 15?
• It means 2/3 x 15
• It could mean anything. It is helpful if you
think of a situation such as
• Mike ate 2/3 of 15 cookies.
• Susie took 2/3 of her 15 marbles to school.
• The dog ran 2/3 of its 15 laps around the yard.

8
Multiplying Fractions
• Multiplying fractions is easier than adding or
subtraction because you dont need to find common
denominators. YAY!!!!!!
• Just multiply straight across.
• Multiply numerators together.
• Then, multiply denominators together.

9
A Few Examples
• Example 1 2/3 X 4/5
• Example 2 9/2 X 3/7
• Example 3 2 1/6 X 3/2
• Example 4 5 X 2/7

10
Examples
• Example 5
• ¾ 7/8
• Example 6
• 5 1/3 9 ½
• Example 7
• 6(1 2/5)

11
Make Life Easier!! Cross Reduce
• When multiplying, you can simplify your factors
by cross reducing.
• Examples
• 6/35 5/24
• 2/15 3/18
• 1/8 (4/5)

12
Practice Problems
• Glencoe Textbook
• Page 255 - 1-6
• Partners

13
Dividing Fractions and Mixed Numbers
14
Discovery Education Videos
Dividing Fractions
2 minute 4 minute mark
Dividing Fractions Example
15
What in the World is a Reciprocal?
• When two fractions are multiplied together and
their product is 1.
• AKA inverting or flipping a number
• Examples
• The reciprocal of ½ is _______.
• The reciprocal of 1 ¾ is _______.
• The reciprocal of 8 is ________.

16
Rules for Dividing Fractions
• STEP 1 Keep the first fraction the same
• STEP 2 Change the "" sign to "x"
• STEP 3 Invert the second fraction
• (Use its reciprocal)
• STEP 4 Multiply.
• STEP 5 Simplify, if needed.
• Example
• ¼ ½ changes to

17
Algorithm Dividing Fractions
• Remember these three words
• KEEP, CHANGE, RECIPROCAL
• First fraction remains the same (KEEP)
• Division symbol is changed to multiplication
(CHANGE)
• Last fraction is changed to its reciprocal
(RECIPROCAL)
forget to cross reduce if possible)

18
Some Examples
• Example 1
• Example 2
• Example 3
• Example 4

Dont forget to cross reduce if possible ONLY
when multiplying!
19
Examples
• Example 5
• Example 6
• Example 7

Dont forget to cross reduce if possible ONLY
when multiplying!
20
Practice Problems Practice Section of your
notebook
• Glencoe Textbook
• Page 267 - 1-8
• Partners

21
Card and Domino Multiplication
• Each partner draws one (1) card and one (1)
domino
• Card whole number
• Domino fraction
• Multiply your two numbers together (individually)

22
Sources
• http//www.helpwithfractions.com/dividing-fraction
s.html
• accessed 11/25/03
• http//mathforum.org/library/drmath/view/58170.htm
l
• accessed 11/25/03
• http//school.discovery.com/homeworkhelp/webmath/f
ractions.html
• accessed 11/25/03
• Van De Walle, J.A. (2001). Elementary and middle
school mathematics. New York Longman.