x²-2xy-2x+y²+2y+1
=（x²-2xy+y²)-2(x-y)+1
=(x-y)²-2(x-y)+1
=[(x-y)-1]²
=(x-y-1)²你是怎么将-2x+2y化成-2(x-y)的？-2x+2y=-2(x-y)提取-2