\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}
	Centre de gravit\'e d'un triangle \par
	\bigskip
	\begin{tikzpicture}
		\coordinate[label=left:$A$] (A) at (0,3);
		\coordinate[label=right:$B$]  (B) at (-2,0);
		\coordinate[label=above:$C$]  (C) at (2,-1);
		\draw (A)--(B)--(C)--cycle;
		\coordinate[label=below:$C_1$] (mAB) at ($(A)!0.5!(B)$);
		\coordinate[label=above right:$A_1$] (mBC) at ($(B)!0.5!(C)$);
		\coordinate[label=left:$B_1$] (mCA) at ($(C)!0.5!(A)$);
		\draw (C)--(mAB);
		\draw (A)--(mBC);
		\draw (B)--(mCA);
		\coordinate (G) at (intersection of A--mBC and B--mCA) ;
		\fill (G) circle (0.5mm) node[below right,red] {$G$};
		\foreach \p in {A,B,C,mAB,mBC,mCA}
			\fill (\p) circle (0.5mm);
	\end{tikzpicture}
\end{document}

