Euclidean Metric is Metric/Proof 2

Proof
We have that the Euclidean metric on $\mathcal A$ is defined as:


 * $\displaystyle d_2 \left({x, y}\right) = \left({\sum_{i \mathop = 1}^n \left({d_{i'} \left({x_i, y_i}\right)}\right)^2}\right)^{\frac 1 2}$

where $x = \left({x_1, x_2, \ldots, x_n}\right), y = \left({y_1, y_2, \ldots, y_n}\right) \in \mathcal A$.

Proof of $M1$
So axiom $M1$ holds for $d_2$.

Proof of $M2$
Let:
 * $(1): \quad z = \left({z_1, z_2, \ldots, z_n}\right)$
 * $(2): \quad$ all summations be over $i = 1, 2, \ldots, n$
 * $(3): \quad d_{i'} \left({x_i, y_i}\right) = r_i$
 * $(4): \quad d_{i'} \left({y_i, z_i}\right) = s_i$.

Thus we need to show that:
 * $\displaystyle \left({\sum \left({d_{i'} \left({x_i, y_i}\right)}\right)^2}\right)^{\frac 1 2} + \left({\sum \left({d_{i'} \left({y_i, z_i}\right)}\right)^2}\right)^{\frac 1 2} \ge \left({\sum \left({d_{i'} \left({x_i, z_i}\right)}\right)^2}\right)^{\frac 1 2}$

We have:

So axiom $M2$ holds for $d_2$.

Proof of $M3$
So axiom $M3$ holds for $d_2$.

Proof of $M4$
So axiom $M4$ holds for $d_2$.