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**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.

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**Property of Cross Products: **

Cross multiplication, in 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

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**How to Solve a Proportion:**

When solving a proportion you must cross multiply, then find the missing variable. For example: = H Steps: 7×H = 4× 84 1. Write the cross products. 7H = Multiply 7 H = 336 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. )

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

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**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. )

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