Multiplying and Dividing Fractions

- 7th 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 lets master this stuff!!!

MULTIPLYING FRACTIONS ANDMIXED NUMBERS!

Discovery Education Videos

Multiply Fractions

Multiply Mixed Numbers

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 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).

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 dont 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 27/141 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

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

Dividing Fractions

2 minute 4 minute mark

Dividing Fractions Example

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 ________.

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

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)

Some Examples

- Example 1
- Example 2
- Example 3
- Example 4

Dont forget to cross reduce if possible ONLY

when multiplying!

Examples

- Example 5
- Example 6
- Example 7

Dont 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-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.