1. 3.8 Direct, Inverse, and Joint Variation
Objective:
To solve problems involving direct, inverse, and joint variation.

2. k is also called the constant of proportionality
For example: The more hours you work the more money you get. The two are directly related. As one goes up, so must the other. In this example, what does k represent?

3. Example If y varies directly as x and y = 9 when x is 15,
find y when x = 21.

4. Example If y varies directly as x, and y = 15 when
x = 24, find x when y = 25.

5. Example
If a is directly proportional to b3, and a = 10 when b = 2, find a when b = 4.

6. If y varies inversely as x and y = 4 when x is 12, find y when x = 5.

7. Example If y varies inversely as x, and y = 22 when
x = 6, find x when y = 15.

8. Example If m is inversely proportional to n, and
m = 6 when n = 5, find m when n = 12.

9. If y varies jointly as x and z
If y varies jointly as x and z. Then y = 80 when x = 5 and z = 8, find y when x = 16 and z = 2.

10. Example
If z varies jointly as x and the square root of y, and z = 6 when x = 3 and y = 16, find z when x = 7 and y = 4.

11. Example If r varies jointly as s and t and inversely as u, and
r = 18 when s = 2, t = 3, and u = 4, find s when
r = 6, t = 2 and u = 4.

12. Assignment
3.8 Practice Worksheet #5-10
3.8 pg 194 #13-20