Let three circles be externally tangent in pairs, with parallel diameters A1A2,B1B2,C1C2 (i.e. each of the quadrilaterals A1B1B2A2 and A1C1C2A2 is a parallelogram or trapezoid, which segment A1A2 is the base). Prove that A1B2,B1C2,C1A2 intersect at one point.(Yuri Biletsky ) geometryconcurrencyconcurrenttangent circles