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Geometry Difficulty 7.6 National Olympiad, round 2 Prove it 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.

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=MGMP 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 BPB \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 QBQ \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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