(x-y)^2-2x+2y+1=x^2-2xy+y^2-2x+2y+1然后用双十字相乘=(x-y-1)(x-y-1)=(x-y-1)^2
(X-Y-1)(X-Y-1)
(X-Y-1)^2
原式=(X-Y)^2-2(X-Y)+1=(X-Y-1)^2