In quadrilateral ABCD, we know that a, g, b and h are the perpendicular bisectors of AD, BC, DC, and AC. And they intersect at point O.
Is this a cycle quadrilateral? Where is the center an what is the radius? Use the tool bar to construct the circle. Does the circle goes through all the vertices?
You can move the vertices of the quadrilateral. What do you notice about the measure of OA, OB, OC and OD?

Can you prove your answer? (Can you prove why the point that you recognize as the center is equidistant from all four vertices)