Problem

Source: ELMO Shortlist 2012, A10

Tags: geometry, conics, circumcircle, projective geometry, algebra proposed, algebra



Let A1A2A3A4A5A6A7A8 be a cyclic octagon. Let Bi by the intersection of AiAi+1 and Ai+3Ai+4. (Take A9=A1, A10=A2, etc.) Prove that B1,B2,,B8 lie on a conic. David Yang.