[tex]\it \left.\begin{aligned} \it \Delta ABC \equiv \Delta MNP\\ \\ \Delta MNP \equiv \Delta BCA \end{aligned} \right\}\ \Rightarrow \ \Delta ABC \equiv \Delta BCA \Rightarrow\ AB=BC;\ AC=AB \Rightarrow \\ \\ \\ \Rightarrow \Delta ABC-echilateral;\ \ \ \mathcal{P}=4\ell=4\cdot10=40\ cm[/tex]
