Unghiurile interne, de aceeași parte a secantei sunt suplementare.
[tex]\it \angle1+\angle2=180^o \Rightarrow \angle1=180^o-\angle2\ \ \ \ \ (1)\\ \\ \\ \angle1=4\angle2\ \ \ \ \ (2)\\ \\ \\ (1),\ (2) \Rightarrow 4\angle2=180^o-\angle2 \Rightarrow 4\angle2+\angle2=180^o \Rightarrow \\ \\ \Rightarrow 5\angle2=180^o\Big|_{:5} \Rightarrow \angle2=36^o\ \ \ \ \ (3)\\ \\ \\ (2),\ (3) \Rightarrow \angle1=4\cdot36^o=144^o[/tex]
