We may notice that the numerator and denominator in the following fraction are not aligned. And, to me it does not look good.
How to align the numerator and denominator?
\documentclass{article}
\usepackage{amsmath,mathrsfs,xcolor,mathtools}
\let\xoverline\overline % supply your definition
\begin{document}
\begin{equation}
\dfrac{1-k^{m+n+d}}{1-k}
\end{equation}
\end{document}
The above yields:
Here's a comparison of the proposed solutions; the name next to the fraction doesn't necessarily denote the suggestion author's preference.
\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\begin{document}
\makebox[2in][l]{1. Original}$\dfrac{1-k^{m+n+d}}{1-k}$
\bigskip
\makebox[2in][l]{2. egreg}$\dfrac{1-k^{m+n+d}}{1-k^{\hphantom{m+n+d}}}$
\bigskip
\makebox[2in][l]{3. projetmbc}$\dfrac{1-k^S}{1-k}, \text{ with $S=m+n+d$}$
\bigskip
\makebox[2in][l]{4. Peter Grill}$\dfrac{1-k^{\mathrlap{m+n+d}}}{1-k}$
\end{document}
Take your pick.
Everybody can add here their personal preference.
\frac{1-k^{m+n+d}}{1-k^{\hphantom{m+n+d}}}– egreg Sep 25 '12 at 08:52\dfrac{1-k^{m+n+d}}{1-k\quad}But I'm not sure it looks nicer than the default one. – yo' Sep 25 '12 at 09:42\dfrac{1-k^{m+n+d}}{1-k\hfill}– morbusg Sep 25 '12 at 14:34\dfrac{1-k^S}{1-k}withS=m+n+d? – projetmbc Sep 25 '12 at 15:07\;:\[ \dfrac{1-k^{m+n+d}}{\;\;1-k} \]or\[ \frac{1-k^{m+n+d}}{1-k\;\;} \]– Américo Tavares Sep 25 '12 at 16:04\frac{1-k^{\mathrlap{m+n+d}}}{1-k}. I'd still recommend leaving it as is or using\hphantomas per @egreg's solution. This requires themathtoolspackage to use as is. Otherwise use\rlap{$ ... $}– Peter Grill Sep 25 '12 at 17:05