Align split equation to the margins

Question

How to align a two-lined equation such that the first line left aligns with the surrounding text and the second line right aligns with the surrounding text?

Example (where the pipes denote the left and right edge of the surrounding text):

|                       |
P(x) =
    a + b + c + d + e + f
|                       |

I fiddled around with flalign, multline, and multlined, but did not get it working the way I want. Prepending a \vphantom to the second line allows me to right-align the second line of the equation. Appending a \vphantom to the first line does not left-align the first line (the \vphantom continues to run in the right margin).

Answer 1

You could use the environment multline provided by the package amsmath

\documentclass{article}

\usepackage{amsmath}
\setlength\multlinegap{0pt}

\begin{document}

\begin{multline}
P(x) =  \\
   a + b + c + d + e + f
\end{multline}

\end{document}

The \setlength\multlinegap{0pt} assures no indentation of the first line.

Answer 2

Ypu can define the flmultline environment and its starred counterpart:

\documentclass{article}
\usepackage{amsmath}
\usepackage{lipsum}

\makeatletter
\newenvironment{flmultline}
  {\multlinegap=\z@\start@multline\st@rredfalse}
  {\endmultline}
\newenvironment{flmultline*}
  {\multlinegap=\z@\start@multline\st@rredtrue}
  {\endmultline}
\makeatother

\begin{document}
\lipsum*[2]
\begin{flmultline*}
P(x)=\\
aaaaa+bbbbb+ccccc+ddddd+eeeee+fffff+ggggg+hhhhh+iiiii
\end{flmultline*}
\lipsum*[3]
\begin{flmultline}
P(x)=\\
aaaaa+bbbbb+ccccc+ddddd+eeeee+fffff+ggggg+hhhhh+iiiii
\end{flmultline}
\lipsum[4]
\end{document}

The idea is simple: locally set \multlinegap to zero.