Looking at the diagram above, since OA and OB are radii of the circle, then OA = OB, therefore forms an isosceles triangle inside the circle.
Also, note that since is isosceles, then , this means that the angles at A and B are equal.
In this circle, calculate the size of .
is a straight angle, so will add up to . This is the straight line .
. This is the other angle at .
is isosceles, so and angles in a triangle will add up to .
This means the angles at and will have split between them equally.