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Geometry Difficulty 5.8 AIME, harder Prove it

One, (50 points) In ABC\triangle A B C, AB=AC,OA B=A C, O is the midpoint of BCB C, a circle with OO as the center is tangent to ABA B and ACA C at points EE and FF respectively. On the arc \overparenEF\overparen{E F} of O\odot O, take any point DD, draw the tangent line through DD intersecting ABA B and ACA C at points PP and QQ respectively, and connect CPC P intersecting EFE F at point RR. Prove: QRABQ R \parallel A B.

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Solution

As shown in Figure 2, connect OA,OD,OE,OF,OP,O A, O D, O E, O F, O P, and OQO Q. To prove QR//ABQ R / / A B, we need to prove that
ERRF=AQQF. \frac{E R}{R F}=\frac{A Q}{Q F}.

By Menelaus' theorem, we have
ERRF=EPPAACCF. \frac{E R}{R F}=\frac{E P}{P A} \cdot \frac{A C}{C F}.

It is also easy to see that AOCOFC\triangle A O C \sim \triangle O F C, hence
ACCF=SAOCSOFC=OA2OF2. \frac{A C}{C F}=\frac{S_{\triangle A O C}}{S_{\triangle O F C}}=\frac{O A^{2}}{O F^{2}}.

Therefore, ERRF=EPPAOA2OF2\frac{E R}{R F}=\frac{E P}{P A} \cdot \frac{O A^{2}}{O F^{2}}.
Thus, we need to prove that AQQF=EPPAOA2OF2\frac{A Q}{Q F}=\frac{E P}{P A} \cdot \frac{O A^{2}}{O F^{2}}, which is equivalent to
AQQFAPPE=OA2OF2. \frac{A Q}{Q F} \cdot \frac{A P}{P E}=\frac{O A^{2}}{O F^{2}}.

On the other hand, by the tangent length theorem, we have
AOQ=AOFQOF=12EOF12DOF=12EOD=POE. \begin{array}{l} \angle A O Q=\angle A O F - \angle Q O F \\ =\frac{1}{2} \angle E O F - \frac{1}{2} \angle D O F = \frac{1}{2} \angle E O D = \angle P O E. \end{array}

Similarly, AOP=QOF\angle A O P = \angle Q O F.
Thus, AQQFAPPE=SAOQSQOFSAOPSAOE\frac{A Q}{Q F} \cdot \frac{A P}{P E}=\frac{S_{\triangle A O Q}}{S_{\triangle Q O F}} \cdot \frac{S_{\triangle A O P}}{S_{\triangle A O E}}
=AOsinAOQOFsinQOFAOsinAOPOEsinPOE=OA2OF2. =\frac{A O \sin \angle A O Q}{O F \sin \angle Q O F} \cdot \frac{A O \sin \angle A O P}{O E \sin \angle P O E}=\frac{O A^{2}}{O F^{2}}.

Therefore, the conclusion holds.

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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.