Sum of Roots of Polynomial

Theorem
Let $P$ be the polynomial equation:
 * $a_n z^n + a_{n - 1} z^{n - 1} + \cdots + a_1 z + a_0 = 0$

such that $a_n \ne 0$.

The sum of the roots of $P$ is $-\dfrac {a_{n - 1} } {a_n}$.