General ⇒ Line under the sqrt's index

[Neo_Prando]

Hi, I've just joined this beautiful forum and I have a question for you.
I'm a LaTeX's beginner and I want to modify how the \sqrt[] {} command is displayed adding a line under the root's index. My problem is this: when it is too long (like this: \(\sqrt[a long index 123456789] {some argument} \)) it just float above the blanck but to write complex formulas I need this line for a clearer reading.
I know I could write it using powers and a fraction as index, but I'd like to obtain it whit radicals.
Thank you all.

[MjK]

You could use hacks like:

Code: Select all

\documentclass{article}

\usepackage{mathtools}

\begin{document}
\[ \sqrt[\leftroot{5}\uproot{2}\underline{a long index 123456789}] {some argument} \]
\end{document}

or draw the square root yourself using TikZ.