First circle theorem


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The Seven Theorems


Angle at centre and angle at circumference


First circle theorem:

What's the connection between the angle at the centre, α, and the angle at the circumference, β?

You can use the cursor to move point C, and hence to alter angle α.
You can also move point D around the circumference.
If you move point B, then angle α and the size of the circle both change.
Notice what happens to angle β as you make these changes.

This is a Java Applet created using GeoGebra from www.geogebra.org - it looks like you don't have Java installed, please go to www.java.com

If you need them, the seven circle theorems here

Tim Devereux, Created with GeoGebra