Question

help me to find this

Answer 1

Since, OA || BC and OB is transversal,

∠AOB = ∠CBO [Alternate interior angles]
⇒∠AOB=80°

If an arc subtends an angle 2Ф at the the centre of the circle then, it subtends an an angle of Ф at any point on the circumference of the circle.

Here, arc AB subtends central angle of 80°.
So, it must subtend an angle of 40° at the circumference.

∴∠ACB = 40°