Find the rate of change of the area of a circle per second with respect to its radius r when r = 2.5 cm.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → $5\pi$
A, Area of circle = $\pi r^2$
$⇒\frac{dA}{dr}=2\pi r$
$⇒\left.\frac{dA}{dr}\right|_{r=2.5}=2\pi(2.5)$
$=5\pi$