equal size numerator and denominator

Question

I am trying to size the numerator and denominator of the below equation so that they are equal in size. I have not had much luck suggestions are appreciated.

\begin{equation}
\sum_{i=1}^{n} \frac{\frac{cf_{n}}{(1+i)^n}} {\displaystyle\sum_{i=1}^{n}\frac{cf_n}{(1+i)^n}} , n_{i}
\end{equation}

Answer 1

Probably you had added \displaystyle to get the superscript and subscript of the inner sum operator on top and below the symbol. This also increases the size of the fraction. Instead \limits can be used to move the superscript and subscript of the operator at the same place as in \displaystyle:

\documentclass{article}
\begin{document}
\[
  \sum_{i=1}^{n}
  \frac{\frac{cf_{n}}{(1+i)^n}}
       {\sum\limits_{i=1}^{n}\frac{cf_n}{(1+i)^n}}
  , n_{i}
\]
\end{document}

The following example makes the four math style visible:

  ① \displaystyle
  ② \textstyle
  ③ \scriptstyle
  ④ \scriptscriptstyle

\documentclass{article}

\usepackage{pifont}
\usepackage{amstext}
\usepackage{color}
\newcommand*{\showms}{%
  \mathchoice
    {{\scriptscriptstyle\text{\color{red}\ding{172}}}}%
    {{\scriptscriptstyle\text{\color{red}\ding{173}}}}%
    {{\scriptscriptstyle\text{\color{red}\ding{174}}}}%
    {{\scriptscriptstyle\text{\color{red}\ding{175}}}}%
}

\begin{document}
\[
  \showms\sum_{\showms i=1}^{\showms n}
  \showms\frac{
    \showms\frac{\showms cf_{\showms n}}{\showms (1+i)^{\showms n}}
  }{
    \displaystyle
    \showms\sum_{\showms i=1}^{\showms n}
    \showms\frac{\showms cf_{\showms n}}{\showms(1+i)^{\showms n}}
  }\showms,
  n_{\showms i}
\]
\[
  \showms\sum_{\showms i=1}^{\showms n}
  \showms\frac{
    \showms\frac{\showms cf_{\showms n}}{\showms (1+i)^{\showms n}}
  }{
    \showms\sum\limits_{\showms i=1}^{\showms n}
    \showms\frac{\showms cf_{\showms n}}{\showms(1+i)^{\showms n}}
  }\showms,
  n_{\showms i}
\]
\end{document}

Legend:

  ① \displaystyle
  ② \textstyle
  ③ \scriptstyle
  ④ \scriptscriptstyle

Answer 2

Alternatively from New command in TeX for fraction and Mathfixs package:

\documentclass{article}
\usepackage{mathfixs,amsmath,anyfontsize}
\makeatletter
\newcommand*{\fr}[1]{%
  \fr@aux#1,,\@nil
}
\def\fr@aux#1,#2,#3\@nil{%
  \ensuremath{\frac{#1}{#2}}%
}
\makeatother

\begin{document}
\[ \fr{\mbox{\fontsize{2mm}{2mm}\selectfont 5 - 2} ,\textstyle{\mbox{\fontsize{2mm}{2mm}\selectfont 6 - 1}}} = \fr{20,30} \]
\[\frac{\textstyle{\mbox{\fontsize{2mm}{2mm}\selectfont 5 - 2 }}}{\textstyle{\mbox{\fontsize{2mm}{2mm}\selectfont 6 - 1}} } = \frac{20}{30}\]
\end{document}

Notice the difference in vertical alignment between numerator and denominator when using each of the methods.