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Problem 2246

National Olympiad second round; IMO P1/P4
Geometry Difficulty 7.6 Prove it Asian Pacific Mathematics Olympiad · Asia Pacific Mathematics Olympiad (APMO)

Let Γ\Gamma be the circumcircle of a triangle ABCA B C. A circle passing through points AA and CC meets the sides BCB C and BAB A at DD and EE, respectively. The lines ADA D and CEC E meet Γ\Gamma again at GG and HH, respectively. The tangent lines of Γ\Gamma at AA and CC meet the line DED E at LL and MM, respectively. Prove that the lines LHL H and MGM G meet at Γ\Gamma.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

Let MGM G meet Γ\Gamma at PP. Since ∠MCD=∠CAE\angle M C D = \angle C A E and ∠MDC=∠CAE\angle M D C = \angle C A E, we have MC=MDM C = M D. Thus
MD2=MC2=MG⋅MP M D^{2} = M C^{2} = M G \cdot M P
and hence MDM D is tangent to the circumcircle of △DGP\triangle D G P. Therefore ∠DGP=∠EDP\angle D G P = \angle E D P.
Let Γ′\Gamma' be the circumcircle of △BDE\triangle B D E. If B=PB = P, then, since ∠BGD=∠BDE\angle B G D = \angle B D E, the tangent lines of Γ′\Gamma' and Γ\Gamma at BB should coincide, that is Γ′\Gamma' is tangent to Γ\Gamma from inside. Let B≠PB \neq P. If PP lies in the same side of the line BCB C as GG, then we have
∠EDP+∠ABP=180∘ \angle E D P + \angle A B P = 180^{\circ}
because ∠DGP+∠ABP=180∘\angle D G P + \angle A B P = 180^{\circ}. That is, the quadrilateral BPDEB P D E is cyclic, and hence PP is on the intersection of Γ′\Gamma' with Γ\Gamma.
Figure 1
Otherwise,
∠EDP=∠DGP=∠AGP=∠ABP=∠EBP. \angle E D P = \angle D G P = \angle A G P = \angle A B P = \angle E B P .
Therefore the quadrilateral PBDEP B D E is cyclic, and hence PP again is on the intersection of Γ′\Gamma' with Γ\Gamma.
Similarly, if LHL H meets Γ\Gamma at QQ, we either have Q=BQ = B, in which case Γ′\Gamma' is tangent to Γ\Gamma from inside, or Q≠BQ \neq B. In the latter case, QQ is on the intersection of Γ′\Gamma' with Γ\Gamma. In either case, we have P=QP = Q.

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