1. Using Cross Products Lesson 6-4

2. Cross Products When you have a proportion (two equal ratios), then you have equivalent cross products. Find the cross product by multiplying the denominator of each ratio by the numerator of the other ratio.

3. Example: Do the ratios form a proportion? Check using cross products. 4 12, 3 9 12 x 3 = 36 9 x 4 = 36 These two ratios DO form a proportion because their cross products are the same.

4. Example 2 5 8, 2 3 8 x 2 = 16 3 x 5 = 15 No, these two ratios DO NOT form a proportion, because their cross products are different.

5. Solving a Proportion Using Cross Products Use the cross products to create an equation. Solve the equation for the variable using the inverse operation.

6. Example: Solve the Proportion k 17 = 20 68 Start with the variable. = 68k17(20) Simplify. 68k=340 Now we have an equation. To get the k by itself, divide both sides by 68. 68 k = 5

7. Homework Time