∵ z=√(x²+y²) ==>αz/αx=x/√(x²+y²),αz/αy=y/√(x²+y²)
∴ds=√[1+(αz/αx)²+(αz/αy)²]dxdy=√2dxdy
故 ∫∫zds=∫∫√(x²+y²)*√2dxdy (S是xoy平面上的圆:x²+y²
∵ z=√(x²+y²) ==>αz/αx=x/√(x²+y²),αz/αy=y/√(x²+y²)
∴ds=√[1+(αz/αx)²+(αz/αy)²]dxdy=√2dxdy
故 ∫∫zds=∫∫√(x²+y²)*√2dxdy (S是xoy平面上的圆:x²+y²