Unghiurile interne, de aceeași parte a secantei, sunt suplementare,
adică suma măsurilor lor este egală cu 180°
[tex]\it \left.\begin{aligned}\widehat1+\widehat2=180^o \Rightarrow \widehat1=180^o-\widehat2\\ \\ \widehat1=\dfrac{3}{5}\cdot\widehat2\end{aligned}\right\} \Rightarrow 180^o-\widehat2=\dfrac{3}{5}\widehat2\Big|_{\cdot5} \Rightarrow 900^o-5\cdot\widehat2=3\cdot\widehat2\Rightarrow\\ \\ \\ \Rightarrow8\cdot\widehat2=900^o|_{:4} \Rightarrow 2\cdot\widehat2=225^o \Rightarrow 2\cdot\widehat2=224^o60'|_{:2} \Rightarrow \widehat2=112^o30'[/tex]
