In triangles AOB and DOCAB = CD. (Given)OB = OD. (Given)(Angle)A

This question was asked in Ex 9.3, 6

In triangles AOB and DOC
AB = CD. (Given)
OB = OD. (Given)
(Angle)AOB = (Angle)DOC. (Vertically opposite angle)
∆AOB =~∆DOC
(ar)AOB =(ar)DOC. (SAS rule)