Draw a quadrilateral ABCD inscribed in a circle having centre O.
Since ABCD is a quadrilateral inscribed in a circle, ABCD becomes a cyclic quadrilateral.
∴ The sum of opposite angles of a cyclic quadrilateral is 180∘.
Since, AB is a diameter of a circle, the angle subtended by AB to the circle is right angle.
In ΔABC, ∠BAC+∠ACB+∠ABC=180∘ [by angle sum property of a triangle]