AB is a diameter of a circle with centre O, and C is a point on the circle with $\angle BOC = 70^\circ$. What is $\angle ABC$?

Explanation

1. 01

   OB and OC are radii, so triangle OBC is isosceles.

2. 02

   $\angle OBC = \angle OCB = (180^\circ - 70^\circ) \div 2 = 55^\circ$.

3. 03

   O lies on AB, so $\angle ABC = \angle OBC = 55^\circ$.