Horizontal alignment in flalign

I feel like this will probably be an obvious one, but I've been struggling to keep the horizontal spacing tight in flalign; but with:

\begin{flalign*}
&\int_{-\pi}^{0} f(x)dx                                     &&  <  0     & \\
&- \Big( \int_{-\pi}^{0} \underbrace{f(x)}_{\leq 0}dx \Big) &&  = -(-2)  & \\
&                                                           &&  = 2      &
\end{flalign*}


I'm getting huge blanks between the two members of the expressions. I've tried all kinds of ampersand arrangements, but no luck so far. Any idea?

• flalign is "full length" align so designed to be rather wide, also you have blank cells at the start of each line and before each operator . There is only one alignment point here so should only be one & (with align) but you have four per line. – David Carlisle Sep 20 '15 at 11:53
• If you're using flalign for “flush left align”, don't; that's the job of the fleqn option instead (and possibly of \setlength{\mathindent}{0pt}). – egreg Sep 20 '15 at 16:13

If you absolutely want to align on the left margin for a particular alignment with flalign*, you can insert an alignedat environment. I took the liberty to replace \underbrace with \underbracket (from mathtools), which looks nicer, in my opinion, for short items:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[showframe, nomarginpar]{geometry}
\usepackage{amssymb,mathtools}
\usepackage{amsfonts}

\begin{document}

\begin{flalign*}
&\! \begin{alignedat}{2} & \int_{-\pi}^{0} f(x)dx& & < 0 \\
&- \Big( \int_{-\pi}^{0} \underbracket[0.4pt][0.5ex]{f(x)}_{\leq 0}dx \Big) && = -(-2) \\
& && = 2
\end{alignedat} &
\end{flalign*}

\end{document}


• Ok, great! Agreed, the underbracket does indeed look nicer. Thanks a bunch. – Chris Sep 20 '15 at 13:09