\documentclass[12pt,a4paper]{article}
\usepackage[latin1]{inputenc}

\usepackage{times,courier,fullpage,ngerman}

\parindent 0pt
\parskip 2mm

\begin{document}
\title{Lösung zur Formelsatz-Übung}
\author{K. Knopper}
\date{\today}
\maketitle

\section*{Aufgabe 2}

\begin{displaymath}
c = \sqrt{ a^2 + b^2 }
\end{displaymath}

\begin{center}
\verb|c = \sqrt{ a^2 + b^2 }|
\end{center}

\section*{Aufgabe 3}

\begin{displaymath}
\sum_{x=0}^{n} \frac{ x^2 } { y^3 }
\end{displaymath}

\begin{center}
\verb|\sum_{x=0}^{n} \frac{ x^2 } { y^3 }|
\end{center}

\section*{Aufgabe 4}

\begin{displaymath}
\mbox{f}(x) = \left\{
      \begin{array}{ll}
         1 & \forall x \le 1 \\
         x + \mbox{f}(x-1) & \mbox{sonst}.
      \end{array}
\right.
\end{displaymath}

\begin{verbatim}
\mbox{f}(x) = \left\{
      \begin{array}{ll}
         1 & \forall x \le 1 \\
         x + \mbox{f}(x-1) & \mbox{sonst}.
      \end{array}
\right.
\end{verbatim}
\end{document}