ÖMO 2021 Problem 5

Let $ABCD$ be a convex cyclic quadrilateral with diagonals $AC$ and $BD$ . Each of the four vertixes are reflected across the diagonal on which the do not lie. (a) Investigate when the four points thus obtained lie on a straight line and give as simple an equivalent condition as possible to the cyclic quadrilateral $ABCD$ for it. (b) Show that in all other cases the four points thus obtained lie on one circle.

(Theresia Eisenkölbl)