a)
[tex]\it\mathcal{A}_{ABCD}=2\cdot\mathcal{A}_{ABD} =2\cdot\dfrac{BD\cdot AM}{2}=10\cdot8=80\ cm^2[/tex]
b)
[tex]\Delta ABD\equiv \Delta CDB\ (LLL) \Rightarrow AM=CN\ \ \ (1)\\ \\ AM\perp BD,\ \ CN\perp BD \Rightarrow AM||CN\ \ \ \ \ (2)\\ \\ (1),(2) \Rightarrow AMCN-paralelogram[/tex]
