# Viète's Formulas

## Contents

## Theorem

Let

- $\map P x = a_n x^n + a_{n - 1} x^{n - 1} + \dotsb + a_1 x + a_0$

be a polynomial of degree $n$ over a ring $R$.

Suppose $P$ can be expressed as:

- $\displaystyle \map P x = a_n \prod_{k \mathop = 1}^n \paren {x - z_k}$

where $z_1, \ldots, z_k \in R$.

That is, $z_1, \ldots, z_k$ are roots of $P$, not guaranteed to be unique in general.

Then:

- $\displaystyle a_{n - k} = \paren {-1}^k a_n \sum_{1 \mathop \le i_1 \mathop < \dotsb \mathop < i_k \mathop \le n} z_{i_1} \dotsm z_{i_k}$

for $k = 1, 2, \ldots, n$.

Listed explicitly, supposing $a_n$ is invertible:

- $z_1 + z_2 + \cdots + z_n = -a_{n-1} / a_n$
- $z_1 z_2 + \cdots + z_1 z_n + z_2 z_3 + \cdots + z_2 z_n + \cdots + z_{n - 1} z_n = a_{n - 2} / a_n$
- $\cdots$
- $z_1 z_2 \cdots z_n = \paren {-1}^n a_0 / a_n$

## Proof

Note that the indexing $1 \le i_1 \le \cdots i_k \le n$ represents all possible subsets of $\set {1, 2, \dotsc, n}$ of size $k$ up to order.

It follows from Product of Sums: Corollary that $\map P x$ foils as a sum of powers $x^k$ with coefficients as sums of all products of elements of subsets of $\set {z_1, \dotsc, z_n}$ of complementary size $n - k$, hence equating pairwise with the original coefficients of $\map P x$ obtains Viète's Formulas.

$\blacksquare$

## Examples

### Two Numbers whose Sum is $4$ and whose Product is $8$

Let $z_1$ and $z_2$ be two numbers whose sum is $4$ and whose product is $8$.

Then:

\(\displaystyle z_1\) | \(=\) | \(\displaystyle 2 + 2 i\) | |||||||||||

\(\displaystyle z_2\) | \(=\) | \(\displaystyle 2 - 2 i\) |

## Source of Name

This entry was named for François Viète.

## Sources

- 1981: Murray R. Spiegel:
*Theory and Problems of Complex Variables*(SI ed.) ... (previous) ... (next): $1$: Complex Numbers: Supplementary Problems: Polynomial Equations: $103$