lines.tex

\documentclass{article}
\usepackage{amssymb}
\usepackage{tkz-euclide}

\begin{document}
$$
\begin{tikzpicture}
\draw (0,0) -- (4,0) -- (2,3.46) -- cycle;
\draw (2,1.15) circle (1.15);
\filldraw[black] (2,1.15) circle (2pt) node[anchor=east]{O};
\draw (2,1.15) -- (2,0);
\filldraw[black] (0,0) circle (2pt) node[anchor=east]{A};
\filldraw[black] (2,3.46) circle (2pt) node[anchor=south]{B};
\filldraw[black] (4,0) circle (2pt) node[anchor=west]{C};
\end{tikzpicture}
$$
\end{document}