不是曲麵積分。是積分求旋轉麵.

A=intg(2pi*x*ds)=intg(2pi*x*sqt(1+(dx/dy)^2)dy=intg2pi*x*sgt(1+y^2/x^2)dy=intg(2pi*sqt(x^2+y^2)dy=intg2pi*Rdy from 0 to h, So,A=2piRh.

x^2+y^=R, so.dx/dy=-y/x. So,ds=sqt(1+(dx/dy)^2*dy=...