## 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.
If you need them, the seven circle theorems here
Tim Devereux, Created with GeoGebra |