I'm having a bit of difficulty proving this, but this is what I have so far: Let a circle with a center O and a diameter AD. Let triangle ABC be a triangle with all points on the circle and angle A be greater than 90. Because of III. 20, and BOC is 2 times the size of angle A. This is all I can reason through. I can see that it works on GSP, but I haven't been able to create a synthetic proof.