V = (4/3)PI*r^3,dV/dt = 4PI*r^2*dr/dt = 4PI*r^2*2 = 8PI*r^2 (cm^3/s).r=10cm 时，dV/dt = 8*PI*(10)^2 = 8*3.1415926*100 = 2513.274123 (cm^3/s)其中，PI = 3.1415926
好久没做过这样的题了
