2010 AIME II Problem 15

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Concepts:cyclic quadrilateralangle bisector theoremlaw of sinesarea ratio

Difficulty rating: 3700

15.

In triangle ABC,ABC, AC=13,AC = 13, BC=14,BC = 14, and AB=15.AB = 15. Points MM and DD lie on AC\overline{AC} with AM=MCAM = MC and ABD=DBC.\angle ABD = \angle DBC. Points NN and EE lie on AB\overline{AB} with AN=NBAN = NB and ACE=ECB.\angle ACE = \angle ECB. Let PP be the other point of intersection of the circumcircles of AMN\triangle AMN and ADE.\triangle ADE. Ray APAP meets BC\overline{BC} at Q.Q. The ratio BQCQ\frac{BQ}{CQ} can be written in the form mn,\frac{m}{n}, where mm and nn are relatively prime positive integers. Find mn.m - n.

Solution:

By the angle bisector theorem, AE=132715AE = \frac{13}{27} \cdot 15 and CD=142913,CD = \frac{14}{29} \cdot 13, so EE lies on AN\overline{AN} and DD lies on MC,\overline{MC}, with NE=ANAE=152659=518,MD=CMCD=13218229=1358.NE = AN - AE = \frac{15}{2} - \frac{65}{9} = \frac{5}{18}, \qquad MD = CM - CD = \frac{13}{2} - \frac{182}{29} = \frac{13}{58}.

Since AMPNAMPN is cyclic, ENP=ANP=180AMP=DMP,\angle ENP = \angle ANP = 180^\circ - \angle AMP = \angle DMP, and since AEPDAEPD is cyclic, NEP=180AEP=ADP=MDP.\angle NEP = 180^\circ - \angle AEP = \angle ADP = \angle MDP. Hence triangles ENPENP and DMPDMP are similar, so NPMP=NEMD.\frac{NP}{MP} = \frac{NE}{MD}. By the law of sines in triangles ANPANP and AMP,AMP, whose angles ANP\angle ANP and AMP\angle AMP are supplementary, sinBAQsinCAQ=sinNAPsinMAP=NPMP=5/1813/58=145117.\frac{\sin\angle BAQ}{\sin\angle CAQ} = \frac{\sin\angle NAP}{\sin\angle MAP} = \frac{NP}{MP} = \frac{5/18}{13/58} = \frac{145}{117}.

Comparing the areas of triangles ABQABQ and ACQ,ACQ, which share the cevian AQ,\overline{AQ}, BQCQ=[ABQ][ACQ]=ABsinBAQACsinCAQ=1513145117=725507,\frac{BQ}{CQ} = \frac{[ABQ]}{[ACQ]} = \frac{AB \sin\angle BAQ}{AC \sin\angle CAQ} = \frac{15}{13} \cdot \frac{145}{117} = \frac{725}{507}, which is in lowest terms since 507=3132507 = 3 \cdot 13^2 and 725=5229.725 = 5^2 \cdot 29. Thus mn=725507=218.m - n = 725 - 507 = 218.

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