multiplying and dividing fractions
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Multiplying and Dividing Fractions. 7 th Grade Math August, 2012. 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 let’s master this stuff!!!. - PowerPoint PPT PresentationTRANSCRIPT
Multiplying and Dividing Fractions
7th Grade MathAugust, 2012
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 let’s master this stuff!!!
MULTIPLYING FRACTIONS ANDMIXED NUMBERS!
Discovery Education Videos
A Quick Review: KEY POINT when multiplying or dividing
fractions:
☼☼ Change a mixed number into an improper fraction,
Still simplify your answer.
More on Multiplying Fractions:
The word “of” in a problem usually means multiply!
Here is an example: There are 8 cars in Michael’s 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).
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.
Multiplying Fractions: Multiplying fractions is easier than adding or
subtraction because you don’t need to find common denominators. YAY!!!!!!
Just multiply straight across. Multiply numerators together. Then, multiply denominators together.
A Few Examples: Example #1: 2/3 X 4/5
Answer: 8/15
Example #2: 9/2 X 3/7 Answer: 27/14=1 13/27
Example #3: 2 1/6 X 3/2 Answer: 39/12=3 3/12=3 ¼
Example #4: 5 X 2/7 Answer: 10/7=1 3/7
Examples: Example #5:
¾ • 7/8
Example #6: 5 1/3 • 9 ½
Example #7: 6(1 2/5)
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)
Practice Problems –
Glencoe Textbook Page 255 - #1-6 Partners
Dividing Fractions and Mixed Numbers
Discovery Education Videos
2 minute – 4 minute mark
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 ________.
123
32
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
12
41
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 (Don’t forget to cross
reduce if possible)
Some Examples: Example #1:
Example #2:
Example #3:
Example #4:
65
43
42
38
315
322
416
Don’t forget to cross reduce if possible
ONLY when multiplying!
Examples:
Example #5:
Example #6:
Example #7:
21
43
21
85
32
214
Don’t forget to cross reduce if possible ONLY when
multiplying!
Practice Problems – “Practice Section” of your notebook Glencoe Textbook
Page 267 - #1-8 Partners
“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.
Sources: http://www.helpwithfractions.com/dividing-fractions.htm
laccessed 11/25/03
http://mathforum.org/library/drmath/view/58170.htmlaccessed 11/25/03
http://school.discovery.com/homeworkhelp/webmath/fractions.html
accessed 11/25/03
Van De Walle, J.A. (2001). Elementary and middle school mathematics. New York: Longman.