The volume of sphere is increasing at the rate of 1200c.cm/sec. the rate increase in its surface area when the radius is 10cm is

Answer 1

dv/dt = 4πr^2 dr/dt
da/dt = 8πr dr/dt

(dv/dt)/(da/dt) = r/2
so, when r=10, da/dt = 240 cm^2/s