The area of a circle is $A=\pi r^2$. The rate of change of the area with respect to the radius $r$, at $r=5$ cm, is

1. Area: $A=\pi r^2$. 2. Differentiate w.r.t. $r$: $\dfrac{dA}{dr}=2\pi r$. 3. Put $r=5$: $\dfrac{dA}{dr}=2\pi(5)=10\pi$. _Source: syllabus Mathematics Ch 6 "Application of Derivatives", p.1_