In Fig. 16.206, O is the centre of the circle and ∠BDC=42°. The measure of ∠ACB is
∠BDC = 42°
∠ABC = 90° (Angle in a semi-circle)
In 
ABC,
∠ABC + ∠BAC = 42° (Angles on the same segment)
90o + 42o + ∠ACB = 180°
∠ACB = 48°
30
In Fig. 16.206, O is the centre of the circle and ∠BDC=42°. The measure of ∠ACB is
∠BDC = 42°
∠ABC = 90° (Angle in a semi-circle)
In ABC,
∠ABC + ∠BAC = 42° (Angles on the same segment)
90o + 42o + ∠ACB = 180°
∠ACB = 48°