%%
%% The LaTeX Companion, 2ed (second printing August 2004)
%%
%% Example 8-8-8 on page 512.
%%
%% Copyright (C) 2004 Frank Mittelbach, Michel Goossens,
%%    Johannes Braams, David Carlisle, and Chris Rowley
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3
%% of this license or (at your option) any later version.
%%
%% See http://www.latex-project.org/lppl.txt for details.
%%

\documentclass{ttctexa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{111.0pt}

\StartShownPreambleCommands
\usepackage{amsmath,bm}
\StopShownPreambleCommands
\begin{document}
$ \bm{\sqrt[2]{x \times \alpha}} $  but
$ \bm{\sqrt[2]{x \mathbin{\mbox{\unboldmath$\times$}} \alpha}} $
  or the similar
$ \bm{\sqrtsign}{\bm{x} \times \bm{\alpha}} $
\end{document}