下列等式中，不成立的是（）A．yx-xy=y2-x2xyB．x2-2xy+y2x-y=x-yC．xyx2-xy=yx-yD．x2-y2

Question

下列等式中，不成立的是（　　）

A． (y)/(x) -(x)/(y) =(y2 -x2)/(xy)	B． (x2 -2xy+y2)/(x-y) =x-y
C． (xy)/(x2 -xy) =(y)/(x-y)	D． (x2 -y2)/(x-y) =x-y

Answer 1

A、 (y)/(x) -(x)/(y) = (y2)/(xy) - (x2)/(xy) = (y2 - x2)/(xy) ，故A成立，不合题意；
B、 (x2 -2xy+ y2)/(x-y) = ((x-y)2)/(x-y) =x-y，故B成立，不合题意；
C、 (xy)/(x2 -xy) = (xy)/(x(x-y)) = (y)/(x-y) ，故C成立，不合题意；
D、 (x2 - y2)/(x-y) = ((x+y)(x-y))/(x-y) =x+y，故D不成立，符合题意．
故选D．