dxdy=.dy v2(v)ao dx v,(v)ax revi(), ydy-Ce(i(n),y)dy ScB O(x, y)dy-JcE O(x, y)dy d Y1 CR O(x, y)dy+e(x,y)dy x=v2(y) h. 2(x, y)dy P 同理可证 dxdy= P(x, y)dx Ddx xQ dxdy dy xQ yy dc D     =  ( ) ( ) 21    = − dc dc Q( ( y), y)dy Q( ( y), y)dy  2  1   = − 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 ( , ) y x od ( ) 2 x =  y D c CE ( ) 1 x =  y