Answer:
arc ACD = 120°
Step-by-step explanation:
Angle ACD = 120°
From cyclic quadrilaterals, opposite angles are supplementary.
angle ABD + angle ACD = 180
120 + ACD = 180
ACD = 60
Let the centre be point O.
Then arc ACD is the same as the angle AOD
From chord properties,
angle ABD = angle AOD /2
or
AOD = 2 * ABD = 2*60 = 120°
