Question

A rock is thrown in a pond, and creates circular ripples whose radius increases at a rate of 0.2 meter per second. What will be the value of dA/dt where A is the area (in square meter) of the circle after 5 seconds?

Answer 1

A = πr^2
dA/dt = 2πr dr/dt
= ....

You are given dr/dt and
you know that after 5 seconds the radius will be 5s * .2m/s = 1 m
so just sub it in.

Answer 2

@mathhelper
I got 0.4pi m^2/sec