1. Lesson 80: direct variation as a ratio, inverse variation as a ratio

2. When a problem states that variable X varies directly as variable Y, we know that the relationship implied is X = kY And that the first step in solving the problem is to find the value of the constant of proportionality k.

3. There is another way to set up these problems too. X = Y X Y

4. Example: A varies directly as B
Example: A varies directly as B. If A is 50 when B is 5, what is the value of A when B is 7?

5. Answer: A = 70

6. Example: Cost varies directly as the number purchased
Example: Cost varies directly as the number purchased. If 12 can be purchased for $78, how much would 42 cost?

7. Answer: $273

8. If A varies inversely as B, the following equation is implied
If A varies inversely as B, the following equation is implied. A = k/B This statement also implies the inverted ratio. A = B A B Note that both the A’s are on the same side and that both B’s are on the other side. However, the B’s are in the inverted form.

9. Example: Blues vary inversely as yellows squared
Example: Blues vary inversely as yellows squared. If 100 blues go with 2 yellows, how many blues go with 10 yellows?

10. Answer: B = k/Y B = 4 2

11. HW: Lesson 80 #1-30