%%
%% The LaTeX Companion, 3ed
%%
%% Example 8-5-3 on page I-635 in "Basic objects".
%%
%% 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}

%StartShownPreambleCommands
\usepackage{tikz}
\usetikzlibrary {calc}
%StopShownPreambleCommands

\begin{document}
\begin{tikzpicture}
  \coordinate (A) at (0,0);
  \coordinate (B) at (75:3);
  \coordinate (C) at ([shift={(1,-1)}]B);
  \draw (A)node[right]{A} -- (B)node[right]{B}
         -- (C)node[right]{C} -- cycle;
  \draw[dashed,->](B) -- ($(A)!0.5!(C)$);
  \draw[dashed,->](C) -- ($(A)!(C)!(B)$);
\end{tikzpicture}
\end{document}