5) 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?
To find = at r = 8 cm
Area of the circle (A) =
(by chain rule)
Hence, the rate at which the area increases when the radius of the circular wave is 8 cm is