Find a mathematical model that represents the statement. (Determine the constant of proportionality.)
1. y is directly proportional to x. (y = 54 when x = 3.)
2. A varies directly as r2. (A = 9π when r = 3.)
3. y varies inversely as x. (y = 3 when x = 25.)
4. y is inversely proportional to x3. (y = 7 when x = 2.)
5. z varies jointly as x and y. (z = 64 when x = 4 and y = 8.)
6. F is jointly proportional to r and the third power of s. (F = 4158 when r = 11 and s = 3.)
7. P varies directly as x and inversely as the square of y. (P = 28/3 when x = 42 and y = 9.)
8. z varies directly as the square of x and inversely as y. (z = 6 when x = 6 and y = 4.)