You can adjust all three points. Thales' theorem states that if A, B and C are points on a circle where the line AC is a diameter of the circle, then the angle ∠ABC is a right angle.
credit to Jamohyperturbopro and MTCGreen for various trig scripts that I didn't want to make from scratch. For more information see this video. https://www.youtube.com/watch?v=V5jQI8V3nsM Or go here. http://en.wikipedia.org/wiki/Thales%27_theorem
