How to display compound fractions with subscripts correctly?

Question

Which is the correct way to represent a compound fraction in an equation environment? With correct I mean, a way so that the representation would give the same font dimension for all the numerators and denominators. Consider the following image:

I would like, for example, that w_{VDW} would have the same dimension as of H_{W,VDW}. Is this the typographically correct choice? What would you suggest?

I have tried \sfrac but it seemed worse.

Have you read Fractions with large elements? The \ddfrac macro may be what you're looking for. — Mico, Jun 08 '16 at 16:06
personally I'd let the numerator and denominator be smaller but it's your choice but Mico's answer in the referenced question should produce what you want. — David Carlisle, Jun 08 '16 at 16:11
@Mico: your suggestion produces the requested result. Thank you! — Andrea, Jun 08 '16 at 16:17
Without amsmath (and \dfrac), one can always use the \displaystyle\frac{}{} syntax. — Steven B. Segletes, Jun 08 '16 at 16:19
how did you input this? if you use \dfrac on the fractions in both the numerator and denominator of the right-hand side, those fractions should be set "full size". this is no different from using \displaystyle. — barbara beeton, Jun 08 '16 at 16:19

score 4 · Accepted Answer · answered Jun 08 '16 at 16:21

It's mainly a question of style. I'll give you three different strategies.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

First display with \verb|\frac|, in the right hand side
the fractions are smaller
\[
\frac{H_{W,WDV}}{H_{W,TDW}}=
\frac{
  \frac{\Delta_{VDW}}{w_{VDW}}
}{
  \frac{\Delta_{TDW}}{w_{TDW}}
}
\]
and second display with \verb|\dfrac|, in the right
hand side the fractions are larger
\[
\frac{H_{W,WDV}}{H_{W,TDW}}=
\frac{
  \,\dfrac{\Delta_{VDW}}{w_{VDW}\mathstrut}\,
}{
  \,\dfrac{\mathstrut \Delta_{TDW}}{w_{TDW}}\,
}
\]
Finally, a third display with the slashed form
\[
\frac{H_{W,WDV}}{H_{W,TDW}}=
\frac{
  \Delta_{VDW}/w_{VDW}
}{
  \Delta_{TDW}/w_{TDW}
}
\]
\end{document}

score 1 · Answer 2 · answered Jun 08 '16 at 16:57

And a fourth display, with the \mfrac (~80 % of \displaystyle) from nccmath. Picking up egreg's example:

\documentclass{article}
\usepackage{nccmath}

\begin{document}

First display with \verb|\frac|, in the right hand side
the fractions are smaller: \[
\frac{H_{W,WDV}}{H_{W,TDW}}=
\frac{
  \frac{\Delta_{VDW}}{w_{VDW}}
}{
  \frac{\Delta_{TDW}}{w_{TDW}}
}
\]
and second display with \verb|\dfrac|, in the right
hand side the fractions are larger: \[
\frac{H_{W,WDV}}{H_{W,TDW}}=
\frac{
  \,\dfrac{\Delta_{VDW}}{w_{VDW}\mathstrut}\,
}{
  \,\dfrac{\mathstrut \Delta_{TDW}}{w_{TDW}}\,
}
\]
Finally, a third display with \verb|\mfrac| in the right-hand side as a compromise: \[
\frac{H_{W,WDV}}{H_{W,TDW}}=
\frac{
  \,\mfrac{\Delta_{VDW}}{w_{VDW}\mathstrut}\,
}{
  \,\mfrac{\mathstrut \Delta_{TDW}}{w_{TDW}}\,
}
\] \end{document}

How to display compound fractions with subscripts correctly?

2 Answers2