圆柱体体积=底面积*高=3.14*R^2*h
圆柱体侧面积=底面周长*高=3.14*2*R*h
圆柱体体积/圆柱体侧面积=[3.14*R^2*h]/[3.14*2*R*h]
=R^2/2R
=R/2