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Geometry Difficulty 6.6 National olympiad Prove it

Let ABCABC be an acute triangle, and let PP be a point located inside the triangle ABCABC. Let DD be the midpoint of the segment [PC][PC], EE the intersection point of the lines (AP)(AP) and (BC)(BC), and QQ the intersection point of the lines (BP)(BP) and (DE)(DE). Show that, if the angles PAC^\widehat{PAC} and PCB^\widehat{PCB} are equal, then sin(BCQ^)=sin(BAP^)\sin (\widehat{BCQ})=\sin (\widehat{BAP}).

Solution

Let RR be the intersection point of the lines (CQ)(C Q) and (AP)(A P). It suffices to prove the equality of the angles (CB,CQ)=(AB,AR)(C B, C Q)=(A B, A R). To do this, it suffices to prove that the points B,C,A,RB, C, A, R are concyclic, that is, (BR,BC)=(AR,RC)(B R, B C)=(A R, R C). Since (AR,AC)=(CP,CB)(A R, A C) = (C P, C B), it suffices to prove that (BR,BC)=(CP,CB)(B R, B C)=(C P, C B), hence that the lines (BR)(B R) and (PC)(P C) are parallel.
For this last point, one can use that if X=(CP)(BR)X=(C P) \cap(B R) then (X,D,P,C)(X, D, P, C) is a harmonic division. One can also do without projective geometry as follows:
We use Ceva's theorem in the triangle PCEP C E, since the lines (PB)(P B), (DE)(D E), and (RC)(R C) are concurrent at QQ, with R(PE)R \in(P E) and B(CE)B \in(C E). We deduce that

1=CBEBPDCDRERP=CBEBRERP 1=\frac{C B}{E B} \cdot \frac{P D}{C D} \cdot \frac{R E}{R P}=\frac{C B}{E B} \cdot \frac{R E}{R P}

that is,

REEB=RPCB \frac{R E}{E B}=\frac{R P}{C B}

By applying the converse of Thales' theorem to the triangles BREB R E and CPEC P E, we deduce from this last equality that the lines (BR)(B R) and (PC)(P C) are indeed parallel, which concludes.
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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.