Question

A stone is dropped into a quiet lake and waves move in circles at the speed of
5 cm/s.
At the instant when the radius of the circular wave is 8 cm, how fast is
the enclosed area increasing?

Answer 1

@terenzreignz

Answer 2

These questions are eerily similar to each other :D
We're looking for \[\Large \frac{dA}{dt}\]
given that...
\[\Large \frac{dr}{dt}=5\\\Large A= \pi r^2\]

So...\[\huge \frac{dA}{dt}=\frac{dA}{dr}\times \left.\frac{dr}{dt}\right]_{r=8}\]

Answer 3

Thank you sir