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\begin{document}
\begin{de} 
ماتریس سه قطری $n\times n$:
\begin{equation} 
\[T_{n}=\left[\begin{array}{*{5}{c}}
d_{1} & a_{1} & 0 & \ldots & 0 \\
b_{1} & d_{2} & a_{2} & \ddots & \vdots \\
0 & \ddots & \ddots & \ddots & 0 \\
\vdots & \ddots & b_{n-2} & d_{n-1} & a_{n-1} \\
0 & \ldots & 0 & b_{n-1} & d_{n} \\
\end{array}\right]\]
\end{equation}
\end{de}
\end{document}