Solution
= Solution
The matrix $A$ fixes the first coordinate and restricts to one half of the rotation generator on the last two coordinates. Hence
$$
\boxed{e^{xA}=\begin{pmatrix}
1&0&0\\
0&\cos(x/2)&-\sin(x/2)\\
0&\sin(x/2)&\cos(x/2)
\end{pmatrix}}.
$$