Question

The radius of a circular puddle is growing at a rate of 15 cm/s.
(a) How fast is its area growing at the instant when the radius is 30 cm?

Answer 1

The area of a circle when the radius is r,
$$A=\pi r^2$$

differentiating wrt t- time,

$$\frac{dA}{dt}=\pi \frac{dr^2}{dt}=\pi 2r\frac{dr}{dt}$$

substitute the values

Answer 2

it says thats not right :(

Answer 3

can you post your steps that you've tried?