[tex]AC = diagonala \\ Observam \:ca: \\ \frac{AB}{AC} = \frac{BC}{AD} = \frac{AC}{CD}\;\;\;adica: \\ \frac{8}{12} = \frac{6}{9} = \frac{12}{18} \\ => \;\;\; \Delta ABC \;\;\;asemenea\;cu\;\;\;\Delta CAD \\ => \;\;AB \;\;||\;\;CD \\ =>ABCD \;\;\;\text{este un trapez cu bazele AB si CD}[/tex]