00 d v200)80 dxdy= dy vi(y)ax =w2(,y)y-(m),y [ang(x,yd-上Q(x,y)d YIu Q(x,y+」Q(x,y)d CBE y2(y) t o(x, y)dy L 同理可证 aP dxdy=k, P(x,y)dxdx xQ dxdy dy xQ yy dc D     =  ( ) ( ) 21    = − dc dc Q( ( y), y)dy Q( ( y), y)dy  2  1 y x od ( ) 2 x =  y D c CE ( ) 1 x =  y   = − CBE CAE Q(x, y)dy Q(x, y)dy   = + CBE EAC Q(x, y)dy Q(x, y)dy  = L Q ( x, y )dy 同理可证   =  − L D dxdy P x y dx yP ( , )