[tex]\it a)\ ABCD-dreptunghi \Rightarrow \begin{cases} AB=CD\\ AD=BC\\(laturi\ opuse)\end{cases}\stackrel{(C.C.)}{\Longrightarrow}\ \Delta ABD\equiv\Delta CDB\ \ \ (*)\\ \\ \\ b)\ \ \ (*)\Rightarrow \mathcal{A}_{ABD}=\mathcal{A}_{CDB}=\dfrac{\mathcal{A}_{ABCD}}{2}[/tex]
[tex]\it c)\ \ \mathcal{A}_{ABCD}=L\cdot \ell=AB\cdot AD\\ \\ \mathcal{A}_{ABD}=\dfrac{\mathcal{A}_{ABCD}}{2}=\dfrac{AB\cdot AD}{2}[/tex]
