Circle theorems

A tangent and a radius meet at 90 degrees

Angles at the centre of the circle is twice the angle of the circumference

Angles in semi-circle are 90 degrees

Angles in same segment are equal

Opposite angles in a cyclic quadrilateral add to 180 degrees

Tangents from the same point are the same length

The angle between a tangent and a chord is equal to the angle in the alternate segment

The perpendicular from the centre of a chord bisects the chord