∠BAC = 85° ∠BCA = 75° ∠ABC = 180° – 85° – 75° = 20° Angle subtended by an arc at the centre is twice to that subtended at any point on the circumference. ∴ 2∠ABC = ∠AOC ∴ ∠OAC = 40° In ΔOAC, OA = OC = radii ∴ ∠OAC = ∠OCA ∴ 2∠OAC = 180° – 40° = 140° ⇒ ∠OAC =