question_answer 5)

A stone is droped 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:

Let r is the radius and A be the area of the circle.       Given :                      ?.(1)       Now A =                [By (1)]                   Hence rate of increase of area of circle is