Suppose ABC is a triangle with circumcenter O. Point D is the reflection of A with respect to BC. Suppose ℓ is the line which is parallel to BC and passes through O. The line through B and parallel to CD meets ℓ at B1. Lines CB1 and BD intersect at point B2. The line through C parallel to BD and ℓ meet at C1. Finally, BC1 and CD intersects at point C2. Prove that points A,B2,C2,D lie on a circle. geometryIndonesiaRMO2022circumcirclegeometric transformationreflection