∠AOB = 2 × ∠ACB

⇒ ∠AOB = 2 × 50°

⇒ ∠AOB = 100°

OA =OB [Radius of the circle]

∴ ∠OAB = ∠OBA _________________ (i)

In triangle AOB,

∠OAB + ∠OBA + ∠AOB = 180°[Sum of angles of triangle]

⇒ 2 ∠OAB + 100° = 180°[From equation (i)]

⇒ 2 ∠OAB = 80°

⇒ ∠OAB = 40°