Multiplying and Dividing Fractions 7 th Grade Math August, 2013.

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Presentation transcript:

Multiplying and Dividing Fractions 7 th Grade Math August, 2013

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: “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? 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: Multiplying fractions is easier than adding or subtraction because you don’t need to find common denominators. YAY!!!!!! Just multiply straight across. Multiply numerators Then, multiply denominators

A Few Examples: Example #1: 2/3 X 4/5 Answer: 8/15 Example #2: 9/2 X 3/7 Answer: 27/14=1 13/14 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 Example #5: 5 1/3 9 ½ Answer: 50 2/3

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)

What in the World is a “Reciprocal”? Reciprocal: When two fractions are multiplied 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 ________.

Algorithm: Dividing Fractions Remember these three words: KEEP, CHANGE, RECIPROCAL KEEP First fraction CHANGE Division to multiplication RECIPROCAL of last fraction Multiply and Simplify

Some Examples: Example #1: Example #2: Example #3: Example #4: Don’t forget to cross reduce if possible ONLY when multiplying!

“Card and Domino Multiplication/Division ” 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!