Question

prove that, if the polynomial f(x) has a factor (x-a), then f(a)=0. how do i prove it?

Answer 1

If f(x) is a polynomial with factor (x-a), then there is another polynomial q(x) such that
f(x) = (x-a)q(x).

Given that, you should have no difficulties now showing that f(a) = 0.