# Lesson 6-8: Cross Multiplication for Solving and Checking Proportions  In this lesson, we will learn how to use cross multiplication to solve proportions and check if two ratios form a proportion. Cross multiplication in a

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## About Lesson 6-8: Cross Multiplication for Solving and Checking Proportions

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Slide1+Lesson 6-8: Cross Multiplication Objective: We will learn to use cross multiplication to solve a proportion. We will use cross multiplication to check whether two ratios form a proportion.

Slide2+Cross multiplication,  i n a proportion, is the product of one numerator and the other denominator.  Property of Cross Products:    If two ratios form a proportion, the cross products are equal. If 2 ratios have equal cross products, they form a proportion.   Example 1:   Decide whether each pair of ratios forms a proportion.    You have to multiply the numerator with the denominator of the    other fraction; and vice versa.  105   =                  =105        3×35 =  5 ×21        105  =  105 If the two products are equal, they form a proportion. When the cross products are not equal, the ratios do not form a proportion.    v    

Slide3+How to Solve a Proportion:  For example :         4    =       H                          Steps:                       7           84                       7×H = 4× 84                                                                    1. Write the cross products .                   7H    =   336                        2. Multiply                          7 H    =    336                       7             7                         3. To find the value of the variable, undo the                                                                         multiplication by dividing to both sides.                          H   =   48                                                                       4. Divide       (You have to think : The opposite of multiplication is division, so I have to divide the number that is with the variable and do the same for both sides of the problem.  Then I cancel the opposites, and bring down what I have left. )   When solving a proportion you must cross multiply, then find the missing variable.

Slide4+Your Example : Solve the proportion.          2    =    3       4        N

Slide5+Homework:  Textbook Pg. 311 #s 7-10 and 15-22.    (Hint: In the first group answer yes or no, but you must show your work.  In the second set of problems you must find the value of the variable by cross multiplying. )