First circle theorem:[br][br]What's the connection between the angle at the centre, α, and the angle at the circumference, β?[br][br]You can use the cursor to move point B, and hence to alter angle α.[br]You can also move point D around the circumference.[br]If you move point C, then angle α and the size of the circle both change.[br]Notice what happens to angle β as you make these changes

If you need it, this theorem is set out [url=http://www.timdevereux.co.uk/maths/geompages/8theorem.php#th1]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