For triangles, △ADOand △BCO,
OA=OB ..... (given)
DO=CO ..... (given)
∠AOD=∠BOC ..... (vertically opposite angles)
∴△ADO≅△BCO .... (SAS criterion for congruency) (1 mark)
Hence,
AD=BC (corresponding parts)
∠ADO=∠BCO
or ∠ADB=∠BCA ....(corresponding parts) (1 mark)