If $D \in \mathcal{M}_n(\mathbb{C})$ is a diagonal matrix whose diagonal coefficients are $d_0, \ldots, d_{n-1}$, show that $\|D\| = \max_{0 \leq i \leq n-1} |d_i|$.
If $D \in \mathcal{M}_n(\mathbb{C})$ is a diagonal matrix whose diagonal coefficients are $d_0, \ldots, d_{n-1}$, show that $\|D\| = \max_{0 \leq i \leq n-1} |d_i|$.