Hello stackExchange users!
"Prove that 4 points $A, B, X, Y$, no 3 collinear, are concyclic if and only if $\measuredangle XAY = \measuredangle XBY$
(Where $\measuredangle$ stands for directed angle $\mod 180^\circ$)"
I'm quite confused with this one, how does a proof look like that this actually matches the "normal" cyclic quadrilateral theorem? This is part of the book Euclidean Geometry in Mathematical Olympiads by Evan Chen.
Thanks in advance! :)