# How to achieve no indentation for a single equation?

I'd like to have a single equation without indentation at all. I know I can set it globally with

\setlength{\mathindent}{0cm}


But I don't know how to return to the default value after the equation.

Here my minimal example:

\documentclass[a4paper,12pt]{article}
\usepackage[fleqn]{amsmath}
\begin{document}

\noindent
Default indentation:
$$a+b=42$$
%
Now I want no indention for a single equation:
\setlength{\mathindent}{0cm}
$$a+b=42$$
%
Now I'd like to get the default again:
$$a+b=42$$
%
But how?

\end{document}

-

\documentclass[a4paper,12pt]{article}
\usepackage[fleqn]{amsmath}
\begin{document}

\noindent
Default indentation:
$$a+b=42$$
%
Now I want no indention for a single equation:
{\setlength{\mathindent}{0cm}
$$a+b=42$$}%
%
Now I'd like to get the default again:
$$a+b=42$$
%
But how?

\end{document}

-
...even the basic principles are unknown to me. Thanks! I will accept as soon as possible. – thewaywewalk Aug 11 '14 at 16:18
@thewaywewalk almost all tex assignments are local so any settings are lost at the next } or \end{zzz} – David Carlisle Aug 11 '14 at 16:40
thanks, good to know! – thewaywewalk Aug 11 '14 at 17:10