In the figure, 
ABCD is a cyclic quadrilateral. Seg AB is a diameter. If ∠ADC = 120˚, complete the following activity to find measure of ∠ BAC.
ABCD is a cyclic quadrilateral.
∴ ∠ADC + ∠ABC = 180°
∴ 120˚ + ∠ABC = 180°
∴ ∠ABC = 
But ∠ACB = .
......angle in semicircle In ∆ABC,
∠BAC + ∠ACB + ∠ABC = 180°
∴ ∠BAC + 
= 180°
∴ ∠BAC = 
