题目错了
x^2+xy-2y^2+x-4y-2
=(x+2y)(x-y)+(x-4y)-2
=(x+2y+2)(x-y-1)
x^2+xy-2y^2-x+4y-2
=(x+2y)(x-y)-(x-4y)-2
=(x+2y-2)(x-y+1)