Matrix Input

\left( \begin{array}{ccc}
a & b & c \\
d & e & f \\
g & h & i 
\end{array} \right)

\left( \begin{array}{cccc}
1 & 2 & x & n         \\
2 & 4 & 2x & 2n       \\
3 & 6 & 3x & 3n  
\end{array} \right)

\left( \begin{array}{rrcr}
1 & 2 & \cdots & n         \\
2 & 4 & \cdots & 2n        \\
\vdots & \vdots & \ddots & \vdots    \\
m & m\cdot 2 & \cdots & m\cdot n  
\end{array} \right)

\left| \begin{array}{ccc}
\lambda - a & -b & -c \\
-d & \lambda - e & -f \\
-g & -h & \lambda - i
\end{array} \right|

|x| = \left\{ \begin{array}{ll}
         x & \mbox{if $x \geq 0$};\\
        -x & \mbox{if $x < 0$}.
      \end{array} \right.

\begin{array}{lcr}
\mbox{First number} & x & 8 \\
\mbox{Second number} & y & 15 \\
\mbox{Sum} & x + y & 23 \\
\mbox{Difference} & x - y & -7 \\
\mbox{Product} & xy & 120
\end{array}