[tex]\bf 5.\ \ \it \mathcal{A}=\ell^2=81=9^2 \Rightarrow \ell = 9\ cm\\ \\ \mathcal{P}=4\cdot\ell=4\cdot9=36\ cm\\ \\ \\ \bf6.\ \ \it \mathcal{P}=2(L+\ell)=40|_{:2} \Rightarrow L+\ell=20 \Rightarrow L=20-\ell\ \ \ \ (1)\\ \\ L>\ell\ cu\ 2 \Rightarrow L=\ell+2\ \ \ \ \ (2)\\ \\ (1),\ (2) \Rightarrow \ell+2=20-\ell \Rightarrow \ell+\ell=20-2 \Rightarrow 2\ell=18|_{:2} \Rightarrow \ell=9cm\ \ \ \ \ (3)\\ \\ (2),\ (3) \Rightarrow L=9+2 \Rightarrow L=11cm[/tex]
[tex]\it \mathcal{A}=L\cdot\ell=11\cdot9=99\ cm^2[/tex]
