ch 3.7-3.8 homework #14

Question

For the following problem, assume that d varies directly with the square of r.
If d = 99 when r = 3, find d when r = 9.

 

d =?

Answer 1

You have submitted three of these direct variation problems.

I'll work one for you, and expect you to learn the method so that you can solve the others yourself.

 

d = kr² represents the given d varies directly with the square of r. 

 

d = 99 when r = 3, so 99 = k(3²) OR 99 = 9k OR k = 99/9 = 11.

 

Thus, d = 11r²

Now just plug in r = 9, and get d = 11(9²) OR d =11(81) = 891.

 

All direct variation problems are like this; the first statement gives the direct variation (=k times...) formula, the second statement gives you enough data values to calculate what k is, and then the third statement gives you the value of all variables except one, which you then solve for.

 

Now go forth & solve the others!