1. Matrix
$$ \begin{pmatrix}1 & 2 & 3 \4 & 5 & 6 \7 & 8 & 9 \end{pmatrix} \begin{bmatrix}a & b & c \d & e & f \g & h & i \end{bmatrix} \begin{pmatrix} a_{1,1} & a_{1,2} & \cdots & a_{1,n} \ a_{2,1} & a_{2,2} & \cdots & a_{2,n} \ \vdots & \vdots & \ddots & \vdots \ a_{m,1} & a_{m,2} & \cdots & a_{m,n} \end{pmatrix} $$
\begin{pmatrix}
1 & 2 & 3 \\
4 & 5 & 6 \\
7 & 8 & 9
\end{pmatrix}
\begin{bmatrix}
a & b & c \\
d & e & f \\
g & h & i
\end{bmatrix}
\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}