The radius of a circle grows at 2 cm/s. How fast is its area growing when the radius is 5 cm?

Explanation

1. 01

   $A = \pi r^{2}$, so $\frac{dA}{dt} = 2\pi r \frac{dr}{dt}$.

2. 02

   $= 2\pi \times 5 \times 2 = 20\pi$ cm²/s.