So, hier mal eine kleines Beispiel. Es ist zwar etwas aufwändig, sieht aber ganz gut aus.
Code:
\documentclass[a4paper,11pt]{article}
\usepackage{mathstyle}
\usepackage{tabu}
\begin{document}
\begin{tabu} to 5cm{@{}X}
\begin{tabu} to 60 mm {@{}X[-2,r]|X[$$-5,c]|X[$$1,l]}
$\textrm{I}_{a}:$ & \frac{1}{2}x + 1 = 2y & \vert -1 \quad \vert\cdot 4 \\
$\textrm{II}_{a}:$ & \frac{4}{3} = \frac{3}{4}y +2x & \left| -\frac{3}{4}y\right. \\
\end{tabu} \\[.4em] \tabucline{} \\[-.8em]
\begin{tabu} to 60 mm {@{}X[-2,r]|X[$$-5,c]|X[$$2,l]}
$\textrm{I}_{b}:$ & 2x = 8y - 4 & \\[0.4em]
$\textrm{II}_{b}:$ & \frac{4}{3}-\frac{3}{4}y = 2x & \\
\end{tabu} \\[.7em] \tabucline{} \\[-.8em]
\begin{tabu} to 70 mm {X[-2,r]X[$$-5,c]X[$$2,l]}
$\textrm{I}_{b} = \textrm{II}_{b}:$ & 8y - 4 = \frac{4}{3}-\frac{3}{4}y & \\
\end{tabu}
\end{tabu}
\medskip\hrule\medskip
\begin{tabu} to 5cm{@{}X}
\begin{tabu} to 60 mm {@{}X[-2,r]|X[$$-5,r]@{ }X[$$-5,l]|X[$$1,l]}
$\textrm{I}_{a}\!:$ & \frac{1}{2}x + 1 &= 2y & \vert -1 \quad \vert\cdot 4 \\
$\textrm{II}_{a}\!:$ & \frac{4}{3} &= \frac{3}{4}y +2x & \left| -\frac{3}{4}y\right. \\
\end{tabu} \\[.4em] \tabucline{} \\[-.8em]
\begin{tabu} to 60 mm {@{}X[-2,r]|X[$$-5,r]@{ }X[$$-5,l]|X[$$2,l]}
$\textrm{I}_{b}\!:$ & 2x &= 8y - 4 & \\[0.4em]
$\textrm{II}_{b}\!:$ & \frac{4}{3}-\frac{3}{4}y &= 2x & \\
\end{tabu} \\[.7em] \tabucline{} \\[-.8em]
\begin{tabu} to 70 mm {X[-2,r]X[$$-5,c]X[$$2,l]}
$\textrm{I}_{b} = \textrm{II}_{b}\!:$ & 8y - 4 = \frac{4}{3}-\frac{3}{4}y & \\
\end{tabu}
\end{tabu}
\end{document}
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