(1)（xy-x²)除以xy分之x²+y²-2xy 乘 x²分之x-y
=x(y-x)×xy/(x-y)²×(x-y)/x²
=-y
(2)先化简：（x²-2x分之x+2 - x²-4x+4分之x-1)除以 x分之x-4
=[(x+2)/x(x-2)-(x-1)/(x-2)²]×x/(x-4)
=[(x-2)(x+2)-x(x-1)]/x(x-2)²×x/(x-4)
=(x-4)/x(x-2)²×x/(x-4)
=1/(x-2)²
（3）先化简：x²-1分之x²-2x+1 除以 x²+x分之x-1
=(x-1)²/(x+1)(x-1)×x(x+1)/(x-1)
=x