Let $\mathbf{a} \in \mathbb{R}^3$, and let $Q$ be any $3 \times 3$ rotation matrix such that $Q \mathbf{a}=\mathbf{e}_3$. (a) Show, using the notation of (7.25), that $R_\theta=Q^T Z_\theta Q$ represents rotation around a by angle $\theta$.
(b) Verify this formula in the case $\mathbf{a}=\mathbf{e}_2$ by comparing with (7.26).