If a snowball melts so that its surface area decreases at a rate of 4 cm2/min, find the rate at which the diameter decreases when the diameter is 12 cm.

Answer 1

a = 4 pi r^2 = pi d^2
da/dt = 2 pi d dd/dt
Now just solve
-4 = 2 pi (12) dd/dt

Answer 2

-1/12pi