Confused

[Dinky_Bell]

Suppose c is inversely proportional to the spuare of d. If c=45 when d=3, find the constant of proprtionality and write the formula for c as a function of d. Use this formula to find c when d=5.


Did anyone understand that????

 

[wjm11]

Suppose c is inversely proportional to the spuare of d. If c=45 when d=3, find the constant of proprtionality and write the formula for c as a function of d. Use this formula to find c when d=5.

Suppose c is inversely proportional to the square of d:

c = k/(d^2)

If c=45 when d=3, find the constant of proportionality:

45 = k/(3^2) = k/9
k = 45*9 = 405

Write the formula for c as a function of d:

c = 405/(d^2)

Next plug in 5 for d.

 

[stapel]

The method outlined by wjm11 is exactly the way to work these exercises: Translate into the basic variation equation, plug in the given data and solve for the variation constant, and then answer the actual question.

For future reference:

. . . . ."y varies directly as x": y = kx

. . . . ."y varies inversely as x": y = k/x

. . . . ."y varies jointly with x and z": y = kxz

If you have these relations memorized, then this sort of exercise should become fairly formulaic.

Eliz.