\documentclass[12pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[french]{babel}
\usepackage{amsmath,amssymb}

\begin{document}

\section*{Mon document}

Voici un exemple : $f(x) = \frac{x^2+1}{2x-3}$

\end{document}