Convergents are Best Approximations/Corollary

Corollary to Convergents are Best Approximations
Let $x$ be an irrational number.

Let $\sequence {p_n}_{n \mathop \ge 0}$ and $\sequence {q_n}_{n \mathop \ge 0}$ be the numerators and denominators of its continued fraction expansion.

Each convergent $\dfrac {p_n} {q_n}$ is a best rational approximation to $x$.

That is, for any rational number $\dfrac a b$ such that $1 \le b \le q_n$:
 * $\size {x - \dfrac {p_n} {q_n} } \le \size {x - \dfrac a b}$

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

Proof
Assume otherwise:
 * $\exists \dfrac a b$ such that $1 \le b \le q_n$

and:
 * $\size {x - \dfrac {p_n} {q_n} } > \size {x - \dfrac a b}$

Then:

which contradicts the result of Convergents are Best Approximations.