The radius of a circle is a line, which starts at the centre of the circle and ends on a point on the circumference. The radius is half the size of the diameter.

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.

- Question
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.