在括号内填上适当的项：x²-xy+y-1=x²-( )

分解因式,得x²-xy+y-1=(x²-1)-(xy-y)=(x+1)(x-1)-y(x-1)=(x-1)(x-y+1)但我不知道如何填这个空要填的话只能填x²-xy+y-1=x²-(xy-y+1)要帮不上忙抱歉啦