\documentclass[12pt]{article}
 \textheight15cm \textwidth11cm \parindent0mm \parskip 1ex plus2ex minus0.8ex
\begin{document}
\arraycolsep 3em
\arrayrulewidth 0.5mm
\doublerulesep 0.2ex
\setlength{\arraycolsep}{3em}
\renewcommand{\arraystretch}{1.4}

\begin{eqnarray}
 f(x)                & = & \cos x \\ 
 f'(x)               & = & -\sin x \\ 
 \int_{0}^{x} f(y)dy & = & \sin x
\end{eqnarray}

\begin{displaymath}
{\bf X} = \left( \begin{array}{ccc}
                  x_{11} & x_{12} & \ldots \\ 
                  x_{21} & x_{22} & \ldots \\ 
                  \vdots & \vdots & \ddots
                 \end{array}
          \right)
\end{displaymath}

\begin{displaymath}
  x = \left\{
        \begin{array}{ll} y   & \mbox{if $y>0$} \\ 
                          z+y & \mbox{otherwise}
        \end{array}
      \right.
\end{displaymath}

\begin{displaymath}
  x = \cases    {
    \begin{array}{ll} y   & \mbox{if $y>0$} \\ 
                      z+y & \mbox{otherwise}
    \end{array} }
\end{displaymath}

\end{document}
