For all $(M, N) \in (\mathcal{M}_n(\mathbb{R}))^2$, we denote
$$(M \mid N) = \operatorname{tr}({}^t M N)$$
Prove that this defines an inner product on $\mathcal{M}_n(\mathbb{R})$. Explicitly express $(M \mid N)$ in terms of the coefficients of $M$ and $N$.