\documentclass[a4paper,12pt]{report}
\usepackage{german,pstricks,pst-plot,pst-node}
\usepackage[latin1]{inputenc}
\begin{document}

Näherung der Ableitung durch eine Sekante.

\psset{unit=6mm}
\begin{center}\pspicture(0,-1)(10,8)\small
  \psaxes{->}(10,8)
  \psplot[linewidth=2\pslinewidth,linecolor=red]
    {0.5}{8}{x dup mul 6 64 div mul 2 add}
  \psline[linestyle=dotted](2,0)(2,3)
  \psline[linestyle=dotted](6,0)(6,6)
  \psline[linewidth=.5\pslinewidth](1,2.1)(7,6.6)
  \psline[linewidth=.5\pslinewidth](1,1.25)(7,5.75)
  \rput[l](5.8,5.8){\rnode{S}{}}
  \rput[l](8,7){\rnode{Se}{Sekante}}
  \rput[l](7.5,5.5){Tangente $\approx f'(x)$}
\endpspicture\end{center}

\nccurve[angleA=120,angleB=80]{->}{Se}{S}

\end{document}