1. The problem statement, all variables and given/known data I have got a question that might be easy for you. But, not for me. I have attached a pic. Please tell me if the quadrilateral inside the circle is a cyclic quadrilateral or not. If it is, then please explain how to prove that it's opposite angles sum up to 180 degrees 2. Relevant equations I believe it won't require any equation. 3. The attempt at a solution No idea !