https://math.stackexchange.com/questions/4636595/given-a-convex-quadrilateral-abcd-find-the-missing-angle?rq=1

 

, George Tsikaloudakis, https://www.facebook.com/photo/?fbid=122096857881164960&set=pcb.122096858427164960

 

Angel Silva Palacios, Geometria Super Top, https://www.facebook.com/photo?fbid=1683039299322921&set=gm.2131596027610755&idorvanity=830782544358783

 

https://artofproblemsolving.com/community/c4t48f4h3726553_solving_without_using_trigonometry

 

A vanished problem

 

https://artofproblemsolving.com/community/c4t48f4h3725945_geo_problem

 

Math.stackexchange, https://math.stackexchange.com/questions/1128917/prove-that-the-circumcenter-of-triangle-piq-is-on-the-hypotenuse-ac?rq=1