x²-4xy+3y²-x-y-2
=(x-y)(x-3y)-(x+y)-2然后十字相乘
(x-y) + 1
(x-3y) - 2
—————————
+(x-3y) -2(x-y)=-(x+y)
原式=(x-y+1)(x-3y-2)