(x²-y²-2x+2y)/(x+ y-2) =((x+y)(x-y)-2(x-y))/(x+ y-2)
=(x-y)((x+y)-2)/(x+ y-2)
=x-y
① （x,y)→（2,0） =2
②（x,y)→（0,2） =-2