matrix on latex code example

Example 1: matrix latex

\begin{bmatrix}
1 & 2 & 3 \\
a & b & c \\
a & b & c \\
\end{bmatrix}

Example 2: matrix latex

\begin{equation*}
A_{m,n} = 
\begin{pmatrix}
a_{1,1} & a_{1,2} & \cdots & a_{1,n} \\
a_{2,1} & a_{2,2} & \cdots & a_{2,n} \\
\vdots  & \vdots  & \ddots & \vdots  \\
a_{m,1} & a_{m,2} & \cdots & a_{m,n} 
\end{pmatrix}
\end{equation*}