
\documentclass{article}
  \usepackage[width=6cm]{geometry}
  \usepackage{array}
  \pagestyle{empty}

\newenvironment{disarray}{%
 \everymath{\displaystyle\everymath{}}\array%
}{%
 \endarray%
}

\begin{document}
\(
\begin{disarray}{|l|}
 y =\sum_{i=0}^{n} i^2 \\
 y =\sum_{i=0}^{n} (i+1)^2 \\
 y =\sum_{i=0}^{n} \bigl(i^2+i+1\bigr)^2 \\
\end{disarray}
\)
\end{document}
