Explicație pas cu pas:
A,O,C-puncte coliniare
<AOC=180°(1)
<BOC=120°(2)
(1),(2)=><AOB=60°
<ABO=80°(3)
<BOA=60°(4)
(3),(4)=><OAB=40°
∆BAD 1. BA=CD
}. 2.AD=BC. }=>∆BAD=∆DCB=>
∆DCB 3.<BAD=<BCD
=><BDC=<DBA=80°