Question: ab is the perpendicular bisector of cd?
Answer: Usually, one makes all the arcs with the same radii. It is simpler that way. If that is the case, then the figure is a rhombus, and we know that the diagonals of a rhombus are perpendicular bisectors of each other. ... Since C and D are on the perpendicular bisector of AB, CD is the perpendicular bisector of AB.