Question

The radius r of a circle is increasing at a rate of 8 centimeters per minute. find the rate of change of the area when r = 39 centimeters

Answer 1

Dr/dt= 8cmM-¹
dA/dt = ?
but A= πr²
so dA/dr =2πr
but recall
dA/dt = dA/dr • dr/dt
dA/dt = 2πr • 8cmM-¹
dA/dt = 2×π×39 ×8 = 624cm²M-1