A=πr²
dA/dt=dA/dr×dr/dt
dA/dr=2πr
dA/dt=2πr dr/dt
we are told that the radius is increasing at the rate 3 in/min
dr/dt=3 in/min
we are required to find:
a] (dA/dt) when radius is r=5 inch
dA/dt=2πr dr/dt
dA/dt=2π(5)×3
dA/dt=30π in²/ min
b] (dA/dt) when radius is r=21 in
dA/dt=2πr dr/dt
dA/dt=2π(21)×3
dA/dt=126π in²/min
