1. 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.

2. 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

3. 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. )

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

5. 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. )