[tex]\it Un\ triunghi \ cu\ laturile \ de\ lungimi \ egale \ este\ triunghi \ echilateral. \\ \\
Fie\ \ell-latura\ triunghiului, iar\ \mathcal{P}-perimetrul\ acestuia.
[/tex]
[tex]\it \mathcal{P} = 3\ell \ \textgreater \ \ell\ cu \ 42 \Rightarrow 3\ell=\ell+42 \Rightarrow 3\ell-\ell=42 \Rightarrow 2\ell=42 \Rightarrow
\\ \\
\Rightarrow \ell=42:2 \Rightarrow \ell = 21\ cm[/tex]
