The Total solution for NCERT class 6-12
In ΔDOC and ΔBOA,
AB || CD, thus alternate interior angles willbe equal,
∴∠CDO = ∠ABO
Similarly,∠DCO = ∠BAO
Also, for the two triangles ΔDOC and ΔBOA,vertically opposite angles will be equal;∴∠DOC = ∠BOA
Hence, by AAA similarity criterion,ΔDOC ~ ΔBOA
Thus, the corresponding sides areproportional.
DO/BO = OC/OA
⇒OA/OC = OB/OD
Hence, proved.