[tex]\it 1km=100 dam\\ \\ \mathcal{P}=2(L+\ell) \Rightarrow 100=2(L+\ell)\bigg|_{:2} \Rightarrow L+\ell=50 \Rightarrow \ell=50-L\ \ \ \ \ (1)\\ \\ \\ L > \ \ell\ de\ 4\ ori \Rightarrow \ell\ < \ L\ de\ 4\ ori \Rightarrow \ell=\dfrac{L}{4}\ \ \ \ \ \ (2)\\ \\ \\ (1),\ (2) \Rightarrow \dfrac{L}{4}=50-L\bigg|_{\cdot4} \Rightarrow L=200-4L\bigg|_{+4L} \Rightarrow 5L=200\bigg|_{:5} \Rightarrow L=40\ dam[/tex]
