Circle Theorems

The angle subtended by a chord at the centre is twice the size of the angle subtended by the same chord at the circumference.

The angles subtended by a chord in the same segment are equal.

The angle of a semi-circle is a right angle.

The opposite angles of a cyclic quadrilateral add up to 180⁰

The angle between a tangent and a radius at the point of contact is a right angle.

The two tangents drawn to a circle from a point are equal in length.

The angle between a chord and a tangent is equal to the angle subtended by the same chord in the alternate segment.