[tex]\cfrac{AC}{BC}\implies \cfrac{\stackrel{AC=3+9}{3~~ + ~~9}}{\underset{BC=9}{9}}\implies \cfrac{\stackrel{4}{~~\begin{matrix} 12 \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}~~}}{\underset{3}{~~\begin{matrix} 9 \\[-0.7em]\cline{1-1}\\[-5pt]\end{matrix}~~}}\implies \cfrac{4}{3}\implies 4~:~3[/tex]
