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Finding Proportions using Cross MultiplicationPowerPoint Presentation

Finding Proportions using Cross Multiplication

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Finding Proportions using 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.

Property of Cross Products: If two ratios form a proportion, the cross products are equal. When 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

Cross multiplication, in a proportion, is the product of one numerator and the other denominator.

Example 1:3 ? 5

9 = 15

Example 2: 3 ? 12

5 = 20

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

5. Write answer

(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.

Your Example: Solve the proportion. 2 = 3 4 N X = 16 3

Homework:

In 1-5, answer yes or no, but you must show your work.

Find the value of the variable by using cross multiplication.

1. 3 ? 5

9 = 15

2. 5 ? 2

8 = 3

3. 7 ? 25

9 = 27

4. 6 ? 10

8 = 15

7 ? 20

9 = 27

6. 3 = 11

X 20

7. 4 = 10

9 M

8. Y = 3

10 5

9. 9 = T

1 4

10. 4 = X

7 84

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