a-a^3+4a^2b-4ab^2
=a[1-﹙a²-4ab+4b²﹚]
=a[1-﹙a-2b﹚²]
=a﹙1+a-2b﹚﹙1-a+2b﹚
4x^2-y^2+4y-4
=4x²-﹙y-2﹚²
=﹙2x+y-2﹚﹙2x-y+2﹚