Thứ Sáu, 12 tháng 9, 2014

28-Six Concyclic Points via Antipedal Triangle

Let ABC be a triangle, and D be any point on the plane. d_a,d_b,d_c respectively are three lines perpendicular with DA,DB,DC at A,B,Cd_a meets d_b at C_1d_b meets d_c at A_1d_c meets d_a at B_1. Circle center A_1, radii A_1D meet BC at A_b,A_c. Define B_a,B_c and C_a,C_b cyclically. Prove that six points A_b,A_c,B_a,B_c,C_a,C_b lie on a circle.

http://www.cut-the-knot.org/m/Geometry/SixConcyclicPoints.shtml

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