P1509.26(1) x=(+u) 证 u=x-y 2 lv=x+y I=2f(u)dudu =4f()dg+hv+nf(a)d“b (u+a)(udu+o(a-u)f(uduP150 9.26(1)  = + = − v x y u x y 证：  = − = +  ( ) 21 ( ) 21 y v u x v u 21 J = I f u dudv D = 1 21 2 ( ) =  + +  − − A A u A f u du A u f u du 0 0 ( ) ( ) ( ) ( )     + − − = + u A A A u A f u du dv f u du dv 0 0 0 0 ( ) ( ) u v x y