Points A,B,C and D lie on line l in this order. Two circular arcs C1 and C2, which both lie on one side of line l, pass through points A and B and two circular arcs C3 and C4 pass through points C and D such that C1 is tangent to C3 and C2 is tangent to C4. Prove that the common external tangent of C2 and C3 and the common external tangent of C1 and C4 meet each other on line l.Proposed by Ali Khezeli functiongeometryIranIranian TST