Second circle theorem

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Theorem 4
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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.

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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