Angle at circumference is half that at the centre
Angles in the same segment are equal
Opposite angles in a cyclic quadrilateral add to 180°
The angle at the circumference of a semicircle is 90°
The angle between a tangent and a radius is 90°
The perpendicular from the centre of a circle to a chord bisects the chord.
Alternate Segment Theorem-The angle between a tangent and a chord is equal to the angle in the alternate segment.
Tangents meeting at the same point are the same length