Distance on Real Numbers is Metric

Real Number Line
Let $x, y \in \R$ be real numbers.

Let $d \left({x, y}\right)$ be the distance between $x$ and $y$.

Then $d \left({x, y}\right)$ is a metric on $\R$.

Thus it follows that $\left({\R, d}\right)$ is a metric space.

Proof for Real Number Line
We check the criteria for $d$ being a metric space in turn.


 * M0: $\forall x, y \in X: \left|{x - y}\right| \ge 0$:

This follows from the definition of absolute value.


 * M1: $\forall x, y \in X: \left|{x - y}\right| = 0 \iff x = y$:

Again, this follows from the definition of absolute value.


 * M2: $\forall x, y \in X: \left|{x - y}\right| = \left|{y - x}\right|$:

As $x - y = - \left({y - x}\right)$, it follows from the definition of absolute value that $\left|{x - y}\right| = \left|{y - x}\right|$.


 * M3: $\forall x, y, z \in X: \left|{x - y}\right| + \left|{y - z}\right| \ge \left|{x - z}\right|$

We have $\left({x - y}\right) + \left({y - z}\right) = \left({x - z}\right)$.

The result follows from the Triangle Inequality.