1,(x-2y)(x-y)(x^2+3xy+2y^2)
=(x-2y)(x-y)(x+y)(x+2y)
=[(x-2y)(x+2y)][(x-y)(x+y)]
=(x^2-4y^2)(x^2-y^2)
=x^4-5x^2y^2+4y^4
2,(3x-1)^2-(3x+1)^2
=9x^2-6x+1-9x^2-6x-1
=-12x第一题的=(x-2y)(x-y)(x+y)(x+2y)这步是怎么得到的x^2+3xy+2y^2=(x+y)(x+2y)