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Taiwan IMO Selection Camp

Geometry Difficulty 5.4 AIME, harder Prove it Taiwan

Given a convex pentagon ABCDEABCDE, let point A1A_1 be the intersection of lines BDBD and CECE, let point B1B_1 be the intersection of lines CECE and DADA, and define points C1,D1,E1C_1, D_1, E_1 analogously. Furthermore, let point A2A_2 be the other intersection point of the circumcircle of triangle ABD1ABD_1 and the circumcircle of triangle AEC1AEC_1, let point B2B_2 be the other intersection point of the circumcircle of triangle BCE1BCE_1 and the circumcircle of triangle BAD1BAD_1, and define points C2,D2,E2C_2, D_2, E_2 analogously. Prove that lines AA2,BB2,CC2,DD2,EE2AA_2, BB_2, CC_2, DD_2, EE_2 are concurrent.

Solution

Perform an inversion with center AA and denote inverse point with prime.

Figure 1

Let P≡AC1′∩⊙(AB′B2′)P \equiv AC'_1 \cap \odot(AB'B'_2), Q≡AD1′∩⊙(AE′E2′)Q \equiv AD'_1 \cap \odot(AE'E'_2), R≡B′D1′∩⊙(AB′D′)R \equiv B'D'_1 \cap \odot(AB'D'), S≡E′C1′∩⊙(AC′E′)S \equiv E'C'_1 \cap \odot(AC'E'). Clearly E1′∈⊙(AB′D′)E'_1 \in \odot(AB'D'), B2′∈⊙(AB′C′)B'_2 \in \odot(AB'C').

B′D1′B'D'_1 and B′,C′,E1′,B2′B', C', E'_1, B'_2 are concyclic, so from Reim's theorem we get AR∥C′B2′AR \parallel C'B'_2. Similarly, we can prove that AS∥D′E2′AS \parallel D'E'_2.

Note that A,B′,E′,C1′,D1′A, B', E', C'_1, D'_1 are concyclic, so from Reim's theorem we get C1′D1′∥B2′P∥E2′Q∥D′R∥C′SC'_1D'_1 \parallel B'_2P \parallel E'_2Q \parallel D'R \parallel C'S, hence
{AD1′C′D1′=D1′RD1′B2′=D′C1′C1′PAC1′D′C1′=C1′SC1′E2′=C′D1′D1′Q}⟹AC1′C1′P=AD1′D1′Q⟹C1′D1′∥PQ, \left\{ \begin{array}{l} \dfrac{AD'_1}{C'D'_1} = \dfrac{D'_1R}{D'_1B'_2} = \dfrac{D'C'_1}{C'_1P} \\ \\ \dfrac{AC'_1}{D'C'_1} = \dfrac{C'_1S}{C'_1E'_2} = \dfrac{C'D'_1}{D'_1Q} \end{array} \right\} \Longrightarrow \dfrac{AC'_1}{C'_1P} = \dfrac{AD'_1}{D'_1Q} \Longrightarrow C'_1D'_1 \parallel PQ,
which implies that P,Q,B2′,E2′P, Q, B'_2, E'_2 are collinear and C1′D1′∥B2′E2′C'_1D'_1 \parallel B'_2E'_2.
From Reim's theorem we get B′,E′,B2′,E2′B', E', B'_2, E'_2 are concyclic, so B,E,B2,E2B, E, B_2, E_2 lie on a circle Γ\Gamma, which implies that AA2,BB2,EE2AA_2, BB_2, EE_2 are concurrent at the radical center TT of Γ\Gamma, ⊙(ABD1)\odot(ABD_1), and ⊙(AEC1)\odot(AEC_1). Analogously, we can prove TT lies on CC2CC_2 and DD2DD_2, so AA2,BB2,CC2,DD2,EE2AA_2, BB_2, CC_2, DD_2, EE_2 are concurrent.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.