For every matrix $A$ in $\mathcal{M}_n(\mathbb{K})$, we set $N(A) = \max_{1 \leqslant i \leqslant n}\left(\sum_{j=1}^{n}|a_{i,j}|\right)$. Show that the map $A \mapsto N(A)$ is a sub-multiplicative norm on $\mathcal{M}_n(\mathbb{K})$.
For every matrix $A$ in $\mathcal{M}_n(\mathbb{K})$, we set $N(A) = \max_{1 \leqslant i \leqslant n}\left(\sum_{j=1}^{n}|a_{i,j}|\right)$. Show that the map $A \mapsto N(A)$ is a sub-multiplicative norm on $\mathcal{M}_n(\mathbb{K})$.