1. The radius of a sphere is r inches at time t seconds. Find the radius when the rates of increase of the surface area and the radius are numerically equal.

A
1/4π in
B
1/5π in
C
1/3π in
D
1/8π in
E
1/2π in

10 answers

a = 4πr^2
da/dt = 8πr dr/dt
if the rates are the same,
1 = 8πr, so r = 1/(8π)
So, (D)