How fast is the area growing after three days of spreading?

Question

An oil spill in the gulf of Mexico is spreading out in a circular shape. The radius is spreading at 5 miles per day. How fast is the area growing after three days of spreading?

Answer 1

Area A = πr²; dA/dt = 2πrdr/dt = 2π·3mi·5mi/day = 30πmi²/day

Answer 2

A = pi(r^2)

A'(t) = 2pi(r)r'

A'(1) = 2pi(r)(5)

A'(3) = 3(2pi(r)(5)

A'(3) = 30pi(r) miles per day