[tex]\it \mathcal{P}_{ABC}=AB+BC+AC =18 \Rightarrow 5+5+AC=18 \Rightarrow AC=8\ cm\\ \\ Pentru\ \Delta ABC\ avem: BC=a=5,\ \ AC=b=8,\ \ AB=c=5\\ \\ \mathcal{A}_{ABC}=\sqrt{p(p-a)(p-b)(p-c)}\\ \\ p=\dfrac{\mathcal{P}}{2}=\dfrac{18}{2}=9;\ \ p-a=p-c=9-5=4;\ \ p-b=9-8=1\\ \\ \\ \mathcal{A}_{ABC}=\sqrt{9\cdot4\cdot1\cdot4}=3\cdot2\cdot2=12cm^2[/tex]
[tex]\it \mathcal{A}_{ABCD}=2\cdot\mathcal{A}_{ABC}==2\cdot12=24\ cm^2[/tex]
