In Δ ABC, ∠ BAC = 85° ∠ BCA = 75° ∴ ∠ABC = 180° – 85° – 75° = 20° The angle subtended by an arc of a circle at the centre is double the angle subtended by it at any point on the remaining part of the circle. ∴ ∠AOC = ∠ABC = 40° ∴ OA = OC = radii In DOAC, ∠OAC = ∠OCA (The angles at the base of an isosceles triangle are equal) ∠OAC + ∠OCA = 180° – 40° = 140° ∴ ∠OAC =