x³+ax²+ax+a-1
=x³-1+ax²+ax+a
=(x-1) (x²+x+1)+ a(x²+x+1)
=(x²+x+1)(x-1+a)
x³-x²-y³+y²
=x³-y³-x²+y²
=(x-y)(x²+xy+y²)-(x-y)(x+y)
=(x-y)(x²+xy+y²-x-y)