I've realized that MathJax version 2.7.3 doesn't work so I move back to 2.7.2
Add these line below <header>
\begin{matrix}
A
\end{matrix}
=
\begin{bmatrix}
$Z_{1}+Z_{2}+Z_{3} & -Z_{3} & 0 & 0 & 0 & 0 & 0\\
-Z_3 & Z_3+Z_4+Z_5+Z_6 & -Z_6 & 0 & 0 & 0 & 0\\
0 & 0 & -Z_7 & Z_7+Z_8+Z_9 & -Z_9 & 0 & 0\\
0 & 0 & -Z_7 & Z_7+Z_8+Z_9 & -Z_9 & 0 & 0\\
0 & 0 & 0 & -Z_9 & Z_9+Z_{10}+Z_{11} & -Z_{11} & 0\\
0 & 0 & 0 & 0 & -Z_{11} & Z_{11}+Z_{12}+Z_{13} & -Z_{13}\\
0 & 0 & 0 & 0 & 0 & -Z_{13} & Z_{13}+Z_{14}$
\end{bmatrix}
