% !TeX TXS-program:compile = txs:///pdflatex

\documentclass[margin=5mm]{standalone}
\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[line join=round]
	%points
	\coordinate (O) at (0,0) ; \coordinate (B) at (8.4,0) ;
	\coordinate (C) at (0,2.8) ; \coordinate (A) at (-2.4,-2.4) ;
	\coordinate (E) at (-2.4,0.4) ; \coordinate (F) at (6,0.4) ;
	\coordinate (D) at (6,-2.4) ; \coordinate (G) at (8.4,2.8) ;
	%tracés
	\filldraw[very thick,dashed,gray,fill=lightgray!50] (C) -- (B) -- (A) -- cycle ;
	\draw[very thick] (A) -- (E) -- (F) -- (D) --cycle ;
	\draw[very thick] (E) -- (C) -- (G) -- (F) ;
	\draw[very thick] (D) -- (B) -- (G) ;
	\draw[very thick,dashed] (A) -- (O) -- (B) (O) -- (C) ;
	%textes
	\foreach \Point/\Pos in {O/below,A/below left,B/right,C/above} \filldraw (\Point) circle[radius=2pt] node[\Pos=3pt] {\Point} ;
\end{tikzpicture}

\end{document}