\documentclass[multi=math,ignorerest,border=0.5]{standalone}[2011/12/21]
\usepackage{amsmath}% always a good idea when using math
% Redefine math related macros and environments:
\AtBeginDocument{%
\def\({\begingroup\math}%
\def\){\endmath\endgroup}%
\def\[{\(\displaystyle}%
\def\]{\)}%
\def\displaymath{\math\displaystyle}%
\def\enddisplaymath{\endmath}%
}
\begin{document}
Test
\[
a = \sum_{k=0}^{N} k^2
\]
text
\(
a = \sum_{k=0}^{N} k^2
\)
\end{document}