# Euler-Binet Formula/Corollary 1

## Corollary to Euler-Binet Formula

$F_n = \dfrac {\phi^n} {\sqrt 5}$ rounded to the nearest integer

where:

$F_n$ denotes the $n$th Fibonacci number
$\phi$ denotes the golden mean.

## Proof

By definition of $n$th Fibonacci number, $F_n$ is an integer.

From Euler-Binet Formula:

$F_n = \dfrac {\phi^n - \hat \phi^n} {\sqrt 5} = \dfrac {\phi^n } {\sqrt 5} - \dfrac {\hat \phi^n} {\sqrt 5}$

But $\size {\dfrac {\hat \phi^n} {\sqrt 5} } < \dfrac 1 2$ for all $n \ge 0$.

Thus $\dfrac {\phi^n } {\sqrt 5}$ differs from $F_n$ by a number less than $\dfrac 1 2$.

Thus the nearest integer to $\dfrac {\phi^n } {\sqrt 5}$ is $F_n$.

$\blacksquare$