the radius of a circle is increasing at a constant rate of 0.4 meters per second. what is the rate of increase in the area of the circle at the instant when the circumference is 60pi?

Question

I got to the point of :

dA/dt=(2pi r) dr/dt

I am confused whether I can replace the 2pir with 60 pi since 2pir represents the circumference and if I can put 0.4 as dr/dt?

Answer 1

A = πr²

dA/dt = 2πr(dr/dt) = (60π m/sec)(0.4 m) = 24π m²/sec