$${\frac{{\mathtt{a}}}{{\mathtt{b}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{a}}}{{\mathtt{c}}}} = {\frac{{\mathtt{ac}}}{{\mathtt{bc}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{ab}}}{{\mathtt{bc}}}}$$ ......
$${\frac{\left({\mathtt{ac}}{\mathtt{\,\small\textbf+\,}}{\mathtt{ab}}\right)}{{\mathtt{bc}}}} = {\frac{{a}{\left({\mathtt{c}}{\mathtt{\,\small\textbf+\,}}{\mathtt{b}}\right)}}{{\mathtt{bc}}}}$$
Ich weis nicht was du mit Platzhalter meinst. Wenn du damit eine Variable meinst, dann geht das Bruchrechnen auch mit Platzhalter.
Beispiel: 3/5 = b/c = a
$${\frac{{\mathtt{a}}}{{\mathtt{b}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{a}}}{{\mathtt{c}}}} = {\frac{{\mathtt{ac}}}{{\mathtt{bc}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{ab}}}{{\mathtt{bc}}}}$$ ......
$${\frac{\left({\mathtt{ac}}{\mathtt{\,\small\textbf+\,}}{\mathtt{ab}}\right)}{{\mathtt{bc}}}} = {\frac{{a}{\left({\mathtt{c}}{\mathtt{\,\small\textbf+\,}}{\mathtt{b}}\right)}}{{\mathtt{bc}}}}$$