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Multiplying and Dividing Fractions

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Title: Multiplying and Dividing Fractions


1
Multiplying and Dividing Fractions
  • 7th Grade Math
  • 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,
  • Still simplify your answer.

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
  • Answer 8/15
  • Example 2 9/2 X 3/7
  • Answer 27/141 13/27
  • Example 3 2 1/6 X 3/2
  • Answer 39/123 3/123 ¼
  • Example 4 5 X 2/7
  • Answer 10/71 3/7

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)
  • Then, Multiply and simplify your answer (Dont
    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)
  • Then, compare your answers.

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.
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