极坐标下旋转体侧面积公式

绕极轴的旋转，其面积 = ∫ 2πy ds = ∫ 2π rsinθ √(r^2+r'^2) dθ, where s is arc length.

推导：y = rsinθ; (ds)^2 = (dx)^2 + (dy)^2 = ((-rsinθ+r'cosθ)dθ)^2 + ((rcosθ+r'sinθ)dθ)^2 = （r^2+r'^2)(dθ)^2