00 d, cv2()00 dxdy= dy D Jvi() ax el (), D)dy-O((v),y)dy @(x, y)dy E CBE Q(, y)dy CAE x=yi =00(x,y)+」(x,y) x=v2(y) Q(, y)dy X L 同理可证 -rop 〗a,4b=「P(x,n 上页dx 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