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?

Explanation

Area of circle `A=pi r^2`


`(dA)/dt=2pi r (dr)/dt`


Given `(dr)/dt=5"cm/s"`


`r=8cm`


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