A stone is dropped into the middle of a pond, causing a circular wave. The radius of this

wave is growing at a rate of 4 cm per second.

(a) What is the rate of growth of the area of the wave when its radius is 3 cm?

(b) What is the rate of growth of the area of the wave 2 seconds after the stone is dropped?

you could apply the fact that since it is 4cm per sec then it would be 8 cm per 2 sec, i don't get why we use integration