Factor theorem
As the Remainder Theorem points out, if you divide a polynomial p(x) by a factor x – a of that polynomial, then you will get a zero remainder. Let's look again at that Division Algorithm expression of the polynomial:
p(x) = (x – a)q(x) + r(x)
If x – a is indeed a factor of p(x), then the remainder after division by x – a will be zero. That is:
p(x) = (x – a)q(x)