Third circle theorem:[br][br]Compare the sizes of angles α & β, as[br]you use the cursor to move point C or D around the circumference.[br][br]It doesn't change either angle β- they stay the same![br][br]What is the effect of moving point E?[br]And finally, what happens when you move point B?[br] [br]

If you need it, this theorem is set out [url=http://www.timdevereux.co.uk/maths/geompages/8theorem.php#th3]here[/url] [br]and all the theorems are summarised [url=http://www.timdevereux.co.uk/maths/geompages/8theorem.php]here[br][/url] [br]Tim Devereux, Created with GeoGebra