Question

At most, how many unique roots will a third-degree polynomial function have?

Answer 1

The fundamental theorem of algebra states that a polynomial with degree n has at most n solutions. The "at most" depends on the fact that the solutions might not all be real number.

In fact, if you use complex number, then a polynomial with degree n has exactly n roots.

So, in particular, a third-degree polynomial can have at most 3 roots.

In fact, in general, if the polynomial $p(x)$ has solutions $x_1,\ x_2,\ldots x_n$, then you can factor it as

$p(x) = (x-x_1)(x-x_2)\ldots (x-x_n)$

So, a third-degree polynomial can't have 4 (or more) solutions, because otherwise you could write it as

$p(x)=(x-x_1)(x-x_2)(x-x_3)(x-x_4)$

But this is a fourth-degree polynomial.