Maths Olympiad Prep

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Geometry Difficulty 5.7 AIME, harder Prove it

一、(50 分)As shown in Figure 1, quadrilateral ABCDABCD is inscribed in a circle. The extensions of ABAB and DCDC intersect at EE, and the extensions of ADAD and BCBC intersect at FF. PP is any point on the circle, and PEPE and PFPF intersect the circle at RR and SS, respectively. If the diagonals ACAC and BDBD intersect at TT, prove that RR, TT, and SS are collinear.

Solution

Connect PDP D, ASA S,
RCR C, BRB R, APA P,
SDS D. From
EBREPA\triangle E B R \sim \triangle E P A,
FDSFPA\triangle F D S \sim \triangle F P A,
we have BRPA=EBEP\frac{B R}{P A}=\frac{E B}{E P},
PADS=FPFD \frac{P A}{D S}=\frac{F P}{F D} \text {. }

Multiplying the two equations, we get BRDS=EBFPEPFD\frac{B R}{D S}=\frac{E B \cdot F P}{E P \cdot F D}.
Furthermore, from ECREPD\triangle E C R \sim \triangle E P D, FPDFAS\triangle F P D \sim \triangle F A S, we have CRPD=ECEP\frac{C R}{P D}=\frac{E C}{E P}, PDAS=FPFA\frac{P D}{A S}=\frac{F P}{F A}.
Multiplying these two equations, we get CRAS=ECFPEPFA\frac{C R}{A S}=\frac{E C \cdot F P}{E P \cdot F A}.
From (1) and (2), we have BRASDSCR=EBFAECFD\frac{B R \cdot A S}{D S \cdot C R}=\frac{E B \cdot F A}{E C \cdot F D}.
Thus, BRRCCDDSSAAB=EBBAAFFDDCCE\frac{B R}{R C} \cdot \frac{C D}{D S} \cdot \frac{S A}{A B}=\frac{E B}{B A} \cdot \frac{A F}{F D} \cdot \frac{D C}{C E}.
By Menelaus' theorem, we have
EBBAAFFDDCCE=1 \frac{E B}{B A} \cdot \frac{A F}{F D} \cdot \frac{D C}{C E}=1 \text {. }

From (3) and (4), we get BRRCCDDSSAAB=1\frac{B R}{R C} \cdot \frac{C D}{D S} \cdot \frac{S A}{A B}=1.
Therefore, BDB D, RSR S, and ACA C intersect at one point.
That is, RR, TT, and SS are collinear.

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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.