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\documentclass{scrartcl}
\usepackage{amsmath} 
\usepackage{amssymb} 
\begin{document}
\begin{align}
\begin{split}
\Delta\phi =& \sqrt{bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb\\
&+ aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa}
\end{split}
\end{align}
\end{document}

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\documentclass{scrartcl}
  \usepackage{amsmath}  
 \usepackage{amssymb}   
\begin{document}  \
begin{align} 
 \begin{split}  
\Delta\phi =& \sqrt{bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb\\  &\overline{+ aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa}}  
\end{split}  
\end{align}  
\end{document}

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\documentclass[ngerman]{scrartcl}
\usepackage{amsmath}
\usepackage{babel}
\begin{document}
\begin{align}
\sqrt{\genfrac{}{}{0pt}{}{zaehler}{nenner}}
\end{align}
\end{document}