The radius of a circle meets the tangent of the circle at right angles at the tangent point.
The chord of a circle subtends an angle which is twice that of the angle it makes on the circumference.
The angles a chord makes on the same side of the circumference are equal,
The sum of the diagonally opposite angles of a concylic quadrilateral is equal to 180 degrees.
The sum of all the exterior angles of a concyclic polygon is 360 degrees.