あああああ

いいいい

と

<math>\sum _{k = 1} ^{n} k + 2</math>

$$\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}$$
\end{document}$$