%%
%% The LaTeX Companion, 3ed
%%
%% Example 12-2-5 on page II-237 in "bm --- Making bold".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\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}