Question 1163173
 A spherical snow ball is melting at 120cm3/min. At what rate is the surface area decreasing at when the radius is 15cm?

solution
dv/dt=120cm^3/min  given  
v=4/3(πr3)
dv/dt=4πr2 dr/dt   than dr/dt=120/4πr2
Surface area of sphere  A=4πr2
da/dt=A=4*2πr dr/dt=8πrdr/dt
da/dt=8πrx120/4πr2=240/r
putting r=15
da/dt=240/15=16
rate of change of decreasing  surface are 16cm^2/min