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
- $5\pi$ cm$^2$/cm
- $\pi$ cm$^2$/cm
- $25\pi$ cm$^2$/cm
- $10\pi$ cm$^2$/cm
Show answer and solution
Correct answer: D. $10\pi$ cm$^2$/cm
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_