%%
%% The LaTeX Companion, 3ed
%%
%% Example 8-5-22 on page I-644 in "Transformations and other operations".
%%
%% 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{135.0pt}

  \usepackage{microtype} % for better line breaking

%StartShownPreambleCommands
\usepackage{tikz,lipsum}  \usetikzlibrary{tikzmark}
%StopShownPreambleCommands

\begin{document}
\tikzset{myarrow/.style={->,blue} }
\begin{center}\begin{tikzpicture}[remember picture]
 \draw (0,0)--(3,0) node[midway,below](b){b}
            --(0,2) node[midway,above](c){c}
            --cycle node[midway,left] (a){a};
 \begin{scope}[overlay]
  \node[fill=gray,anchor=text](B) at (pic cs:b)
       {\phantom{$b^2$}};
  \draw[myarrow](b) to [bend right] (B);
  \draw[myarrow](c) -- ([yshift=0.2cm]pic cs:c);
  \draw[myarrow](a) |- ([shift={(-0.1cm,0.1cm)}]pic cs:a);
 \end{scope}
\end{tikzpicture}\end{center}
\lipsum[1][2-4]
\[ \tikzmark{a}a^2 + \tikzmark{b}b^2= \tikzmark{c}c^2 \]
\end{document}
