The hexagon ABLCDK is inscribed and the line LK intersects the segments AD, BC, AC and BD in points M, N, P and Q, respectively. Prove that NL⋅KP⋅MQ=KM⋅PN⋅LQ.
Solution
First solution. Denote s=sin2A^B, t=sin2B^L, u=sin2L^C+A^K, v=sin2C^K, w=sin2D^K and x=sin2L^D+A^K. Then we consecutively have KM⋅PN⋅LQNL⋅KP⋅MQ=NCNL⋅NPNC⋅AKKP⋅KMAK⋅DQMQ⋅LQDQ=vt⋅su⋅uv⋅wx⋅xs⋅tw=1.
Second solution. We choose T∈MN such that ∠NTB=∠ADB=∠ACB and N is between P and T. Then △CNP∼△TNB and therefore (TL+NL)PN=CN⋅BN, whence TL=PNNL⋅KP. Analogously, △DQM∼△TQB implies TL=MQLQ⋅KM and we have the desired result.
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