Răspuns :
Fie succesiunea A, D, B, C în sens trigonometric.
[tex]\it AB\perp CD \Rightarrow m(\stackrel\frown{AD}) =m(\stackrel\frown{DB})=90^o\\ \\ m(\stackrel\frown{PD})=90^o-20^o=70^o\\ \\ m(\widehat{PCB})=m(\stackrel\frown{PB})=m(\stackrel\frown{PD})+m(\stackrel\frown{DB})=70^o+90^o=160^o[/tex]