Magic Constant of Order 3 Magic Square/Proof 2

Proof
Let $M_n$ denote the magic square of order $n$.

By Magic Constant of Magic Square, the magic constant of $M_n$ is given by:
 * $S_n = \dfrac {n \left({n^2 + 1}\right)} 2$

Setting $n = 3$:
 * $S_3 = \dfrac {3 \times 10} 2 = 15$