A stone dropped into a still pond sends out a circular ripple whose radius increases at a constant rate of 2.9 ft/s.

(a) How rapidly is the area enclosed by the ripple increasing when the radius is 2 feet?

The area is increasing at ${\rm ft}^2{\rm /s}$.

(b) How rapidly is the area enclosed by the ripple increasing at the end of 6.8 seconds?

The area is increasing at ${\rm ft}^2{\rm /s}$.