# Correct typesetting of a formula with a long fraction

I have a two-column document and as you can see a long mathematical formula, consisting of a fraction, can't fit in the column width.

What I don't know is what is the right way to typeset such kind of long formulas with respect to typography. If you absolutely have to use a two-column document and you have a long fraction that doesn't fit, how do you typeset it?

• Is it an option that the formula can span both columns? This could be a solution. Alternatively, define parts of the formula in the body text and refer to them by that variable. – 1010011010 Nov 11 '15 at 3:10
• Will it fit if you just break before the long fraction i.e. at the =. It is hard to tell just looking, but that is at least the obvious first step if nothing else. Can't you define the denominator as a sum? It looks to follow a pattern then you just need the summation sign, with limits and one instance of the pattern. – cfr Nov 11 '15 at 3:18
• The first step in typesetting anything is to type the content into your .tex file. If you need help with a later step, please post the results of that first step so that other people can focus on the problematic later step. That is, please post an MWE. – cfr Nov 11 '15 at 3:29
• Use \sum_{i=1}^n P(E|A_i)\cdot P(A_i) – egreg Nov 11 '15 at 7:20
• @1010011010 No that is not an option but generally it is a nice idea. – Adam Nov 11 '15 at 20:31

I suggest you use the \splitdfrac macro of the mathtools package, as well as an align* environment, to typeset the formula.

\documentclass[twocolumn]{article}
\usepackage{mathtools,lipsum}
\begin{document}
\lipsum[1] % filler text
\begin{align*}
P(A_i\mid E ) &= \frac{P(A_i\cap E)}{P(E)}\\
&= \frac{P(E\mid A_i)P(A_i)}{%
\splitdfrac{P(E\mid A_1)P(A_1)+ P(E\mid A_2)P(A_2)}
{ + \dots + P(E\mid A_n)P(A_n)} }
\end{align*}

\lipsum[2] % more filler text
\end{document}


Addendum: As @WillRobertson has pointed out in a comment, encasing the split-line denominator in parentheses may help avoid any ambiguity as to what the denominator is made up of. Doing so lengthens the second line; this can be handled by switching from an align* to a multline* environment for the entire two-line equation.

\documentclass[twocolumn]{article}
\usepackage{mathtools,lipsum}
\begin{document}
\lipsum[1] % filler text
\begin{multline*}
P(A_i\mid E ) = \frac{P(A_i\cap E)}{P(E)}\\
= \frac{P(E\mid A_i)P(A_i)}{%
\biggl(\splitdfrac{P(E\mid A_1)P(A_1)+ P(E\mid A_2)P(A_2)}
{ + \dots + P(E\mid A_n)P(A_n)}\biggr) }
\end{multline*}

\lipsum[2] % more filler text
\end{document}

• But put brackets around the denominator to avoid ambiguity about what the final terms are adding to. – Will Robertson Nov 13 '15 at 2:14
• @WillRobertson - Thanks. I've provided an addendum to address your comment. – Mico Nov 13 '15 at 6:19