Possible Duplicate:
Nice-looking p-th roots
On math.stackexchange I wanted the cube root of a fraction in display mode, and used $$\sqrt[3]{\frac ab}$$ to get it. The 3 comes out very small and low in the root sign. I also thought of $$^3\sqrt{\frac ab}$$ but the 3 comes out too far to the left.
You can use \leftroot (for horizontal shifting), and/or \uproot (for vertical shifting) from the amsmath package; using \scriptstyle you can increase the size of the index (not sure if this is a good idea); preferable to use a superscript instead of the root:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\sqrt[3]{\frac{a}{b}}\qquad \sqrt[\leftroot{-1}\uproot{2}\scriptstyle 3]{\frac{a}{b}}\qquad
\Bigl(\frac{a}{b}\Bigr)^{1/3}
\]
\end{document}
