=(x²-y²)/xy-y(x-y)/x(y-x)
=(x²-y²)/xy+y(x-y)/x(y-x)

=(x²-y²)/xy+y/x
=(x²-y²+y²)/xy
=x/y=(x²-y²)/xy-y(x-y)/x(y-x)=(x²-y²)/xy+y(x-y)/x(y-x)别的都没变-为什么变成+=(x²-y²)/xy-y(x-y)/x(y-x)=(x²-y²)/xy+y(x-y)/x(x-y) 最后一个y-x应变成x-y