[Tex/LaTex] How to achieve no indentation for a single equation

Question

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

\end{document}

Answer 1

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

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

\end{document}