Inthe adjoining figure, ABCD is a cyclic quadrilateral and AOB is diameter. If ∠ ADC = 120° then ∠ BAC =
120°
90°
60°
30°
∠ ABC + ∠ ADC = 180° (opposite angle of of cyclic quad)
∠ ABC + 120° = 180°
∠ ABC = 60°
Also, ∠ ACB = 90° (angle in a semicircle)
In triangle ABC
∠ ABC + ∠ ACB + ∠ BAC = 180°
60° + 90° + ∠ BAC = 180°
∠ BAC = 30°