Second circle theorem

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Theorem 1
Theorem 2
Theorem 3
Theorem 4
Theorem 5
Theorem 6
Theorem 7
The Seven Theorems

Angle in a semicircle

Second circle theorem:

What's the size of the angle α ('angle in a semicircle')?

You can use the cursor to move point D around the circumference;
It doesn't change angle α though!!
It is always 90 degrees.
This theorem is a special case of theorem 1 - 'angle at the centre is twice the angle at the circumference', with the centre angle = 180 degrees.

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

Second circle theorem

Home First page Theorem 1 Theorem 2 Theorem 3 Theorem 4 Theorem 5 Theorem 6 Theorem 7 The Seven Theorems

Angle in a semicircle

Home
First page
Theorem 1
Theorem 2
Theorem 3
Theorem 4
Theorem 5
Theorem 6
Theorem 7
The Seven Theorems