Question

Let A be the area of a circle with radius r. If \displaystyle \frac { dr }{ dt } = 4, find \displaystyle \frac { dA }{ dt } when r = 4

Answer 1

\[8\Pi r\]

Answer 2

\[A=\pi r^2\]
\[A'=2\pi r r'\]

Answer 3

\(r=4, r'=4\) you get
\[2\pi\times 4\times 4 \]

Answer 4

Thanks