\documentclass{article}
\usepackage{german}
\newcounter{saveeqn}
\newcounter{prenum}
\newcommand{\vareqn}[2]{\setcounter{saveeqn}{\value{equation}}%
   \setcounter{equation}{0}\setcounter{prenum}{#1}
   \renewcommand{\theequation}{\arabic{prenum}[#2{equation}]}}
\newcommand{\reseteqn}{\setcounter{equation}{\value{saveeqn}}%
   \renewcommand{\theequation}{\arabic{equation}}} 
\begin{document}
\setcounter{equation}{5}
\vareqn{15}{\alph}
\begin{equation} \frac{a^2 - b^2}{a + b} = a - b \end{equation}
\reseteqn
\begin{equation} \sqrt[n]{\frac{x^n - y^n}{1 + u^{2n}}} \end{equation}
\end{document}
