[Tex/LaTex] \addtolength{\leftskip}{2em} does not apply to equations in the paragraph

equationsindentationparagraphs

I have a paragraph which should be emphasized using left and right skips. Thereby I recognized that it simply ignores equations in this paragraph. Is this reasonable? 

\documentclass{article}
\usepackage{changepage}% http://ctan.org/pkg/changepage
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\lipsum[1]
{
\addtolength{\leftskip}{10em}
\addtolength{\rightskip}{10em}
\lipsum[2]
\begin{equation}
a+b=c\textrm{ is a very very very very very very long equation }a+b=c
\end{equation}
\lipsum[3]
}
\lipsum[4]
\end{document}

During testing you will register no warning. However the equation is broader than the current text's width.

Is there a way without using any environment in order to sqeeze the equation to the surrounding textwidth?

Best Answer

\leftskip and \rightskip are primitive TeX registers that should never be set directly in LaTeX or all list constructs (which is almost all display constructs) will get confused. You should copy the definition of quote in article class and set \leftmargin and \rightmargin 

\documentclass{article}
\usepackage{changepage}% http://ctan.org/pkg/changepage
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}
\lipsum[1]
{
\addtolength{\leftskip}{10em}
\addtolength{\rightskip}{10em}
\lipsum[2]
\begin{equation}
\textrm{This is a long long long long long long equation}
\end{equation}
\lipsum[3]
}


\newenvironment{zz}
               {\list{}{\leftmargin10em \rightmargin\leftmargin}%
                \item\relax}
               {\endlist}

\begin{zz}
\lipsum[2]
\begin{equation}
\textrm{This is a long long long long long long equation}
\end{equation}
\lipsum[3]
\end{zz}


\lipsum[4]
\end{document}
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