What are your solutions to properly place the shortcut 'sym' in symmetric matrices?
\documentclass{book}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\begin{bmatrix}
u,_1+x_3\theta_2,_1-x_2\theta_3,_1&\frac{1}{2}(v,_1-x_3
\theta_1,_1-\theta_3) & \frac{1}{2}(w,_1+x_2\theta_1,_1+\theta_2)\\
\text{Sym.} & {0} & {0} \\
{} & {} & {0}
\end{bmatrix}
\end{equation}
\end{document}
[text edited for a more complex configuration]