Answer:
(b) ∠ADC = 60°
Explanation:
ΔBOC is isosceles and M is the midpoint of BC
then
∠MOC = ∠BOM = 30°
then
∠BOC = 60°
……………………
OC // AB
then
∠ABO = ∠BOC = 60° (alternate-interior angles)
……………………………………………
In ΔOAB ,we have :
OA = OB and ∠ABO = 60°
Then ,
ΔOAB is an equilateral triangle.
then
∠AOB = 60°
……………………………
We get :
∠AOC = ∠AOB + ∠BOC = 60° + 60° = 120°
……………………………………………………
∠ADC = ∠AOC/2 = 120/2 = 60°