fleqn odd behaviour with long formulas

I have a document containing several formulas of different lengths that I need to be aligned on the left. To achieve that I used the fleqn option in \documentclass. It works as expected with most of the formulas, but the longest ones are not indented, as if they weren't written on a line apart:

How can I fix that without breaking my formula in more lines?

MWE

\documentclass[fleqn]{article}

\usepackage{indentfirst}
\usepackage{amsmath}

\begin{document}
\subsection*{Lorem ipsum sit amet}
Short formula: aligned as wanted (the quick brown fox jumps over the lazy dog and the quick brown fox jumps over the lazy dog)
$\begin{split} V_{x} & = \frac{V_{0} - \sum_{m}^{n}Sp + \sum_{m}^{n}Pr} {q^{(n-m)}} = \\ & = 10000 \end{split}$

Longer formula: aligned as wanted (the quick brown fox jumps over the lazy dog and the quick brown fox jumps over the lazy dog)
$\begin{split} \sum_{x}^{y}Sp & = \bigg(1000\frac{q^a-1}{r}q^{b} + 2000\frac{q^{c}-1}{r}q^d\bigg) + \bigg(3000\frac{q^e-1}{r}q + 4000\bigg) = \\ & = 5000 \end{split}$

Even longer formula: oddly aligned (the quick brown fox jumps over the lazy dog and the quick brown fox jumps over the lazy dog)
$\begin{split} \sum_{x}^{y}Pr & = \bigg(100000\frac{q^a-1}{r}q^{b} + 200000\frac{q^{c}-1}{r}q^d\bigg) + \bigg(300000\frac{q^e-1}{r}q + 400000\bigg) = \\ & = 500000 \end{split}$

\end{document}


1 Answer

No odd behavior. The formula doesn't fit the line and LaTeX does its best to accommodate it.

Split the overlong line.

Note. Use \biggl and \biggr, not the unadorned \bigg. Also it's not customary to repeat the equals sign (but your typographic tradition might mandate it; try and avoid it, it's unnecessary).

\documentclass[fleqn]{article}

\usepackage{indentfirst}
\usepackage{amsmath}

\begin{document}
\subsection*{Lorem ipsum sit amet}
Short formula: aligned as wanted (the quick brown fox jumps over the
lazy dog and the quick brown fox jumps over the lazy dog)
$\begin{split} V_{x} & = \frac{V_{0} - \sum_{m}^{n}Sp + \sum_{m}^{n}Pr} {q^{(n-m)}} \\ & = 10000 \end{split}$

Longer formula: aligned as wanted (the quick brown fox jumps over the
lazy dog and the quick brown fox jumps over the lazy dog)
$\begin{split} \sum_{x}^{y}Sp & = \biggl(1000\frac{q^a-1}{r}q^{b} + 2000\frac{q^{c}-1}{r}q^d\biggr) + \biggl(3000\frac{q^e-1}{r}q + 4000\biggr)\\ & = 5000 \end{split}$

Even longer formula: oddly aligned (the quick brown fox jumps over the
lazy dog and the quick brown fox jumps over the lazy dog)
$\begin{split} \sum_{x}^{y}Pr & = \biggl(100000\frac{q^a-1}{r}q^{b} + 200000\frac{q^{c}-1}{r}q^d\biggr) \\ &\qquad+ \biggl(300000\frac{q^e-1}{r}q + 400000\biggr) \\ & = 500000 \end{split}$

\end{document}


• I supposed it was because of something like LaTeX's accomodating system. I was wondering whether there is a way to suggest it a way to do that in another way, such as slightly reducing the space between the signs - I'd really prefer not to split the line. Mar 9 '20 at 9:06
• @Ntakwetet Reducing the space could perhaps help in some situations. With the standard text width, the equation you have doesn't fit no matter what. Mar 9 '20 at 9:08