因式分解x2+y2-xy-x-y+1

x2+y2-xy-x-y+1=1/2(2x2+2y2-2xy-2x-2y+2)=1/2(x2-2xy+y2+x2-2x+1+y2-2y+1)=1/2(x-y)2+1/2(x-1)2+1/2(y-1)2

=2（x+y）-xy-(x+y)+1=(2-1)(x+y)-xy+1=x+y-xy+1

这个题目是这个样子的吗？？？
我有疑问啊