(x+y)(x-y)-(2x+y)(x-y+1)
=x^2-y^2-(2x^2-2xy+2x+xy-y^2+y)
=x^2-y^2-2x^2+2xy-2x-xy+y^2-y
=-x^2+xy-2x-y
如果不懂,祝学习愉快!