## Calculus 8th Edition

(a)$$r=60t$$ (b)$$A= 3600 \pi t^2$$ This function represents the area of growing ripple in terms of time.
(a) The value of the increasing radius equals the velocity times the elapsed time. So we have $$r=v \times t=60t.$$ (b) $$A= \pi r^2 \quad \Rightarrow$$ $$(A \circ r)(t)=A(r(t))=3600 \pi t^2$$