Complex fractions
If you want to write abc, you need to write a^{bc}
instead of a^bc
or a^(bc)
. I prefer this way
\documentclass[10pt]{article}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{textcomp}
\begin{document}
\[\frac{1}{1-\sum_{2}^{n}\cfrac{1}{\left(\frac{r}{R}\right)^{2^{n-1}}}}\]
\end{document}
I propose one of these, based on the medium-size commands from nccmath
(~ 80% of \displaystyle
):
\documentclass[10pt]{article}
\usepackage{amssymb}
\usepackage{mathtools, nccmath}
\begin{document}
\[ \frac{1}{1-\medop\sum_{2}^{n} \frac{1}{\bigl(\mfrac{r}{R}\bigr)^{\! 2^{n-1\mathstrut}}}} \qquad \frac{1}{1-\medop\sum_{2}^{n} \bigl(\mfrac{R}{r}\bigr)^{\!2^{n-1}}}\]
\end{document}
one more variation:
\documentclass[10pt]{article}
\usepackage{nccmath}
\begin{document}
\[
\frac{1}{1-\sum\limits_{2}^{n} \Bigl(\mfrac{r}{R}\Bigr)^{\! -2^{n-1}}}
\]
\end{document}