If r is positive and increasing, for what value of r is the rate of the increase of r^3 twelve times that of r?

Question

A. Cubed root 4
B. 2
C. Cubed root 12
D. 2 sqrt 3
E. 6

Answer 1

let y1 = r
dy1/dr = 1

let y2 = r^3
dy2/dr = 3r^2

when is dy2/dr = 12(dy1/dr) ?
3r^2 = 12(1) ?
r^2 = 4
r = 2