[tex]\it \mathcal{P}=18\dfrac{1}{2}=18,5\ cm\ \ \ \ (1)\\ \\ \\ \ell=\ \dfrac{\ \ 25^{(25}}{100}\ \cdot\ L \Rightarrow \ell=\dfrac{1}{4}\cdot L \Rightarrow \ell=\dfrac{L}{4} \Rightarrow L=4\ell\ \ \ \ (2)[/tex]
[tex]\it \mathcal{P}=2(L+\ell)\ \stackrel{(1),(2)}{\Longrightarrow}\ \ 18,5=2(4\ell+\ell) \Rightarrow 18,5=2\cdot5\ell\bigg|_{:10} \Rightarrow \ell=1,85\ cm[/tex]
