Consider xcm be the radius of the circular wave at time t.  Rate of increase of radius of circular wave =5 cm/sec dtdx​ is positive and =5 cm/sec Consider y be the enclosed area of the circular wave.y=πx2 Rate of change of area =dt dy ​=πdtd​x2 =π2xdtdx​=2πx(5)=10πx Put x=8 cm (Given)dtdy​=10π(8)=80πcm2/sec As, dtdy​ is positive, therefore area of circular wave is increasing at the rate of 80πcm2/sec .