$($a$)$ If $A$ is the area of a circle with radius $r$ and the circle expands as time passes, find $\frac{dA}{dt}$ in terms of $\frac{dr}{dt}.$

$\frac{dA}{dt}=(2r,)\frac{dr}{dt}$

$26m?$

$326\mathrm{.}73\frac{m^2}{s}$