Euclidean Metric on Real Number Line is Metric/Proof 2

Proof
Consider the real number line under the Euclidean metric:
 * $M = \struct {\R, d}$

where $d$ is the distance function given by:
 * $d \tuple {x, y} = \size {x - y}$

Proof of $M1$
So axiom $M1$ holds for $d$.

Proof of $M2$
So axiom $M2$ holds for $d$.

Proof of $M3$
So axiom $M3$ holds for $d$.

Proof of $M4$
So axiom $M4$ holds for $d$.