If the radius of a circle is increasing at the rate of 0.4 cm/sec. Then the rate of increase of its circumference is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 0.8π cm/sec
The circumference is \( C=2\pi r \), so
\( \displaystyle \frac{dC}{dt}=2\pi\frac{dr}{dt} \)
\( \displaystyle =2\pi\times 0.4=0.8\pi\ \text{cm/sec} \)