The angle subtended at the centre of the circle is twice the angle subtended at the edge. 

The angle at the edge of a circle subtended by a diameter is a right angle. 

Opposite angles in a cyclic quadrilateral – where all the vertices are an the edge of the circle – add up to 180 degrees. 

An angle between a tangent and a chord through the point of contact is equal to the angle in the alternate segment. 

Angles in the same segment of a circle subtended from the same chord are equal. 

Angles formed from two points on the circumference are equal to other angles, in the same arc, formed from those two points. 
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