\documentclass[11pt,a4paper]{article}
\usepackage[latin1]{inputenc}
\usepackage{ngerman}
\usepackage{textcomp}
\date{\today}
\author{Physik Praktikum}
\title{Elastizität (1) $E$}

\begin{document}
\markright{Physik Praktikum}
\pagestyle{myheadings}

\maketitle

\section{Fehlerrechnung}
\begin{displaymath}
\Delta  \left< X \right> =
\sqrt{\frac{\sum_1^n(X_m - \left< X \right>)^2}{n(n-1)}}
\end{displaymath}

\subsection{Dehnung}
\begin{displaymath}
\Delta  \left< X \right> =
\pm \sqrt{
\left( \frac{K}{\Delta l q} \Delta \left< l \right> \right)^2 +
\left( \frac{l}{\Delta l q} \Delta \left< K \right> \right)^2 +
\left( \frac{l K}{q} \Delta \left< \Delta l \right> \right)^2 +
\left( \frac{l K}{\Delta l} \Delta \left< q \right> \right)^2 }
\end{displaymath}

\vfill
\begin{flushleft}
\textcopyleft{} Thomas Kuster\\
thomas@fam-kuster.ch
\end{flushleft}
\end{document}