Transformiere die Gleichung so, dass R4 am anfang steht:
$$\left[{\frac{{\mathtt{R4}}}{\left({\mathtt{R3}}{\mathtt{\,\small\textbf+\,}}{\mathtt{R4}}\right)}}\right]{\mathtt{\,\times\,}}\left[{\frac{{\mathtt{U2}}{\mathtt{\,\times\,}}\left({\mathtt{R1}}{\mathtt{\,\small\textbf+\,}}{\mathtt{R2}}\right)}{{\mathtt{R1}}}}\right]{\mathtt{\,-\,}}\left[{\frac{{\mathtt{U1}}{\mathtt{\,\times\,}}{\mathtt{R2}}}{{\mathtt{R1}}}}\right] = {\mathtt{Ua}}$$
Der Taschenrechner macht dann den Rest.
Stelle "Gleichungslöser" ein, gebe die Gleichung ein.
Das Ergebnis sollte so aussehen:
$$\underset{\,\,\,\,{\textcolor[rgb]{0.66,0.66,0.66}{\rightarrow {\mathtt{r4}}}}}{{solve}}{\left(\left[{\frac{{\mathtt{R4}}}{\left({\mathtt{R3}}{\mathtt{\,\small\textbf+\,}}{\mathtt{R4}}\right)}}\right]{\mathtt{\,\times\,}}\left[{\frac{{\mathtt{U2}}{\mathtt{\,\times\,}}\left({\mathtt{R1}}{\mathtt{\,\small\textbf+\,}}{\mathtt{R2}}\right)}{{\mathtt{R1}}}}\right]{\mathtt{\,-\,}}\left[{\frac{{\mathtt{U1}}{\mathtt{\,\times\,}}{\mathtt{R2}}}{{\mathtt{R1}}}}\right]={\mathtt{Ua}}\right)} \Rightarrow {\mathtt{r4}} = {\mathtt{\,-\,}}{\frac{\left({\mathtt{r1}}{\mathtt{\,\times\,}}{\mathtt{r3}}{\mathtt{\,\times\,}}{\mathtt{ua}}{\mathtt{\,\small\textbf+\,}}{\mathtt{r2}}{\mathtt{\,\times\,}}{\mathtt{r3}}{\mathtt{\,\times\,}}{\mathtt{u1}}\right)}{\left({\mathtt{r1}}{\mathtt{\,\times\,}}{\mathtt{ua}}{\mathtt{\,\small\textbf+\,}}\left({\mathtt{\,-\,}}{\mathtt{r2}}{\mathtt{\,-\,}}{\mathtt{r1}}\right){\mathtt{\,\times\,}}{\mathtt{u2}}{\mathtt{\,\small\textbf+\,}}{\mathtt{r2}}{\mathtt{\,\times\,}}{\mathtt{u1}}\right)}}$$
.