roots are complex when the discriminant is negative.
if P is a polynomial in one variable with real coefficients, and a + bi is a root of P with a and b real numbers, then its complex conjugate a − bi is also a root of P.
According to the rational root theorem, if the root is rational, then it should be either 1 or -1. It isn't. If the root is irrational, then its conjugate
a special case (for a single linear factor) of Gauss's lemma on the factorization of polynomials
provides a complete list of possible rational roots of the polynomial equation anxn + an–1xn–1 + ··· + a2x2 + a1x + a0 = 0 where all coefficients are integers