LaTeX Forum

\begin{equation}\label{test}
   \begin{split}
	 		A^2+B^2-C^2&=\underbrace{\left[2\,\left({l_{1}}^2{l_{3}}^2+{l_{2}}^2{l_{3}}^2+{l_{3}}^2{l_{4}}^2+{l_{1}}^2{l_{2}}^2-{l_{1}}^2{l_{4}}^2+{l_{2}}^2{l_{4}}^2\right)-{l_{1}}^4-{l_{2}}^4-{l_{3}}^4-{l_{4}}^4\right]}+\\
	 		&+\underbrace{\left[4\,\left(l_{1}l_{4}{l_{2}}^2+l_{1}l_{4}{l_{3}}^2-l_{4}{l_{1}}^3-l_{1}{l_{4}}^3\right)\right]}\cos\!\psi{}+\\
	 		&+\underbrace{\left[-4\,{l_{1}}^2{l_{4}}^2\right]}\cos^2\!\psi{}
	 \end{split} 
\end{equation}

\documentclass{scrbook}
\usepackage{amsmath}
\begin{document}
\begin{equation}\label{test} 
   \begin{split} 
          A^2+B^2-C^2&=\underbrace{[2\,\left({l_{1}}^2{l_{3}}^2+{l_{2}}^2{l_{3}}^2+{l_{3}}^2{l_{4}}^2+{l_{1}}^2{l_{2}}^2-{l_{1}}^2{l_{4}}^2+{l_{2}}^2{l_{4}}^2\right)-{l_{1}}^4-{l_{2}}^4-{l_{3}}^4-{l_{4}}^4 ]}_{\text{ich bin text}} +\\ 
          &+\underbrace{\left[4\,\left(l_{1}l_{4}{l_{2}}^2+l_{1}l_{4}{l_{3}}^2-l_{4}{l_{1}}^3-l_{1}{l_{4}}^3\right)\right]}\cos\!\psi{}+\\ 
          &+\underbrace{\left[-4\,{l_{1}}^2{l_{4}}^2\right]}\cos^2\!\psi{} 
    \end{split} 
\end{equation}
\end{document}