# Reverse Triangle Inequality/Normed Division Ring

## Theorem

Let $\struct {R, \norm {\,\cdot\,} }$ be a normed division ring.

Then:

- $\forall x, y \in R: \norm {x - y} \ge \big\lvert {\norm x - \norm y} \big\rvert$

## Proof

Let $0$ be the zero of $\struct {R, \norm {\,\cdot\,} }$.

Let $d$ denote the metric induced by $\norm {\, \cdot \,}$, that is,

- $d \tuple {x, y} = \norm {x - y}$

From Metric Induced by Norm is Metric we have that $d$ is indeed a metric.

Then, from the Reverse Triangle Inequality as applied to metric spaces:

- $\forall x, y, z \in R: \big\lvert {\norm {x - z} - \norm {y - z}} \big\rvert \le \norm {x - y}$

Then:

- $\forall x, y \in R: \big\lvert{\norm x - \norm y}\big\rvert = \big\lvert{\norm{x - 0} - \norm{y - 0} }\big\rvert \le \norm {x - y}$.

$\blacksquare$