LaTeX Forum

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 \begin{align}
R_{12}&=\frac{\varrho \cdot l}{A} =\dfrac{0,222
%JB: Achte auf das Komma!
	\dfrac{\Omega\text{\,mm}^2}{\text{m}}\cdot 5\text{m}}{1,885\cdot10^{-3}\text{\,mm}^{2}} \notag \\
R_{12}&=73,54\text{\,m}\Omega\\
R_{12}&= \frac{\varrho \cdot l}{A} =
\frac{\SI{0.222}{\ohm\milli\meter\squared\per\meter} \cdot
\SI{5}{\meter}}%
{\SI{1.885e-3}{\square\milli\meter}}
\end{align}
  
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 \begin{align}
&R_{12}=\frac{\varrho \cdot l}{A} =\dfrac{0,222\dfrac{\Omega\text{\,mm}^2}{\text{m}}\cdot 5\text{m}}{1,885\cdot10^{-3}\text{\,mm}^{2}} \notag \\
&R_{12}=73,54\text{\,m}\Omega 
\end{align}