A stone is dropped into a quiet lake and waves move in circles at a speed of 4 cm/sec. At the instant when the radius of the circular wave is 10 cm, how fast is the enclosed area increasing?

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Given: a stone is dropped into a quiet lake and waves move in circles at a speed of 4 cm/sec. ; To find the instant when the radius of the circular wave is 10 cm, how fast is the enclosed area increasing

Let r be the radius of the circle and A be the area of the circle ; When stone is dropped into the lake waves moves in circle at speed of 4cm/sec.i.e., radius of the circle increases at a rate of 4cm/sec