Convergents are Best Approximations/Corollary

Corollary to Convergents are Best Approximations
Let $\dfrac {p_1} {q_1}, \dfrac {p_2} {q_2}, \ldots$ be the convergents of the irrational number $x$.

Then for any rational number $\dfrac a b$ such that $1 \le b \le q_n$:
 * $\left\vert{x - \dfrac {p_n} {q_n}}\right\vert \le \left\vert{x - \dfrac a b}\right\vert$.

The equality holds only if $a = p_n$ and $b = q_n$.

Proof
Assume otherwise, i.e. that $\exists \dfrac a b$ such that $1 \le b \le q_n$ and $\left\vert{x - \dfrac {p_n} {q_n}}\right\vert > \left\vert{x - \dfrac a b}\right\vert$.

Then:

which contradicts the result of Convergents are Best Approximations.