Inscribed Right Angle
Instructions
Grab and drag the blue inscribed right angle's vertex C and move it around the black part of the circle. Does the move change its intercepted arc (the blue semicircle) or the semicircle's pink diameter?
Now Grab and drag point B to change the size of the circle and the location of the diameter. Does this affect either the inscribed angle measure or its intercepted arc?
Because we know that an inscribed angle is always half its intercepted arc, we can conclude that as long as the inscribed angle measures 90 degrees, its intercepted arc will be 180 degrees, or a semicircle.