Move one or more of the points C, D, E, until you have a right triangle (one of the angles is 90 degrees.)[br]What do you notice about the side opposite the right angle?

The side opposite the right angle goes through the center of the circle. That is, it's a diameter of the circle.[br]Why is this so? Try to prove it. You can also look up a proof.[br]Technical term: a circle is inscribed in a circle if its vertices are all on the circle.