Maths Olympiad Prep

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Geometry Difficulty 5.6 AIME, harder Prove it Bulgaria

The hexagon ABLCDKABLCDK is inscribed and the line LKLK intersects the segments ADAD, BCBC, ACAC and BDBD in points MM, NN, PP and QQ, respectively. Prove that NLKPMQ=KMPNLQNL \cdot KP \cdot MQ = KM \cdot PN \cdot LQ.

Solution

First solution. Denote s=sinA^B2s = \sin \frac{\hat{A}B}{2}, t=sinB^L2t = \sin \frac{\hat{B}L}{2}, u=sinL^C+A^K2u = \sin \frac{\hat{L}C + \hat{A}K}{2}, v=sinC^K2v = \sin \frac{\hat{C}K}{2}, w=sinD^K2w = \sin \frac{\hat{D}K}{2} and x=sinL^D+A^K2x = \sin \frac{\hat{L}D + \hat{A}K}{2}. Then we consecutively have
NLKPMQKMPNLQ=NLNCNCNPKPAKAKKMMQDQDQLQ=tvusvuxwsxwt=1. \frac{NL \cdot KP \cdot MQ}{KM \cdot PN \cdot LQ} = \frac{NL}{NC} \cdot \frac{NC}{NP} \cdot \frac{KP}{AK} \cdot \frac{AK}{KM} \cdot \frac{MQ}{DQ} \cdot \frac{DQ}{LQ} = \frac{t}{v} \cdot \frac{u}{s} \cdot \frac{v}{u} \cdot \frac{x}{w} \cdot \frac{s}{x} \cdot \frac{w}{t} = 1.

Second solution. We choose TMNT \in MN such that NTB=ADB=ACB\angle NTB = \angle ADB = \angle ACB and NN is between PP and TT. Then CNPTNB\triangle CNP \sim \triangle TNB and therefore (TL+NL)PN=CNBN(TL + NL)PN = CN \cdot BN, whence TL=NLKPPNTL = \frac{NL \cdot KP}{PN}. Analogously, DQMTQB\triangle DQM \sim \triangle TQB implies TL=LQKMMQTL = \frac{LQ \cdot KM}{MQ} and we have the desired result.

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