%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-7-16 on page II-204 in "Ghostbusters\texttrademark ".
%%
%% 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{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.3\abovedisplayskip}%
    \setlength\belowdisplayskip{.3\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{mathtools}
%StopShownPreambleCommands

\begin{document}
\[ \sqrt{ \frac{a+b}{x_j} }           \quad
   \sqrt{ \frac{a+b}{\smash{x_j}} }   \quad
   \sqrt{ \frac{a+b}{{}\smash{x_j}} } \quad
   \sqrt{ \frac{a+b}{\smash{x_j+b}} }       \]
\end{document}