いいいい
と
$$ \sum _{k = 1} ^{n} k + 2$$
$$\documentclass{jarticle}
\begin{document}
\[
\left[
\begin{array}{rcl}
a_{000000} & \ldots & a_{n} \\
\vdots & \ddots & \vdots \\
a_0 & \ldots & a_{nnnnnnnn}
\end{array}
\right]
\]
\[
\left[
\begin{array}{lcr}
a_{000000} & \ldots & a_{n} \\
\vdots & \ddots & \vdots \\
a_0 & \ldots & a_{nnnnnnnn}
\end{array}
\right]
\]
$$\documentclass{jarticle} \newcommand{\itbold}[1]{\textbf{\textit{#1}}}
\begin{document}
\begin{eqnarray} \itbold{A} \bullet \itbold{B} &=& (x_{1} \, \itbold{i} + y_{1}\,\itbold{j})
\bullet (x_{2}\,\itbold{i} + y_{2}\,\itbold{j}) \\
&=& x_{1} \, \itbold{i} \bullet x_{2}\,\itbold{i}
+ x_{1} \, \itbold{i} \bullet y_{2}\,\itbold{j}
+ y_{1}\,\itbold{j} \bullet x_{2}\,\itbold{i}
+ y_{1}\,\itbold{j} \bullet y_{2}\,\itbold{j} \\
&=& x_{1} \, x_{2} \cos 0
+ x_{1} \, y_{2} \cos \frac{\pi}{2}
+ y_{1} \, x_{2} \cos \frac{\pi}{2}
+ y_{1} \, y_{2} \cos 0 \\
&=& x_{1} \, x_{2} + y_{1} \, y_{2} \end{eqnarray}
\end{document}$$ \end{document}$$
