Product 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 product of the roots of $P$ is $\dfrac {\paren {-1}^n a_0} {a_n}$.

Proof
Let the roots of $P$ be $z_1, z_2, \ldots, z_n$.

Then $P$ can be written in factored form as:
 * $\displaystyle a_n \prod_{k \mathop = 1}^n \paren {z - z_k} = a_0 \paren {z - z_1} \paren {z - z_2} \dotsm \paren {z - z_n}$

Multiplying this out, $P$ can be expressed as:
 * $a_n \paren {z^n - \paren {z_1 + z_2 + \dotsb + z_n} z^{n - 1} + \dotsb + \paren {-1}^n z_1 z_2 \dotsm z_n} = 0$

where the coefficients of $z^{n - 2}, z^{n - 3}, \ldots$ are more complicated and irrelevant.

Equating powers of $z$, it follows that:
 * $a_n \paren {-1}^n z_1 z_2 \dotsm z_n = a_0$

from which:
 * $z_1 z_2 \dotsm z_n = \dfrac {\paren {-1}^n a_0} {a_n}$