(x-y)(x-y)-(x-y)-12
=(x-y-4)(x-y+3)
把看成(x-y)一个整体
也可以设x-y=t
(x-y)(x-y)-(x-y)-12
=t^2-y-12
=(t-4)(t+3)
=(x-y-4)(x-y+3)