Get both advantage of multline ( spacing) AND equation/multlined (numbering)

Question

I like the spacing of multline but not the numbering (on the last line). I tought for a moment that equation+multlined will do the trick but that is not the case right now. The numbering is OK, but the spacing is far more "centered" on the page and I did not manage to improve it in any way (flushleft/flushright did not work either).

Can anyone help me?

MWE: Compare

\documentclass[a4paper,10pt]{article}

\usepackage{mathtools}

\begin{document}

\begin{multline}
111111111+2222222222\\3333333333333+4444444444\\=XXXXXXXXXXXXXX
\end{multline}

\begin{equation}
\begin{multlined}
111111111+2222222222\\3333333333333+4444444444\\=XXXXXXXXXXXXXX
\end{multlined}
\end{equation}

\end{document}

Answer 1

You get (almost) the same behavior with multlined if you tell it the width you want it to use:

\documentclass[a4paper,10pt]{article}

\usepackage{mathtools}

\begin{document}

\begin{multline}
111111111+2222222222\\3333333333333+4444444444\\=XXXXXXXXXXXXXX
\end{multline}

\begin{equation}
\begin{multlined}[c][.9\displaywidth]
111111111+2222222222\\3333333333333+4444444444\\=XXXXXXXXXXXXXX
\end{multlined}
\end{equation}

\end{document}

The first optional argument tells the alignment with respect to the context, the second is the width. Don't use the whole width, because you have to accommodate the equation number.