Let Γ be the circumcircle of a triangle ABC. A circle passing through points A and C meets the sides BC and BA at D and E, respectively. The lines AD and CE meet Γ again at G and H, respectively. The tangent lines of Γ at A and C meet the line DE at L and M, respectively. Prove that the lines LH and MG meet at Γ.
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Let MG meet Γ at P. Since ∠MCD=∠CAE and ∠MDC=∠CAE, we have MC=MD. Thus MD2=MC2=MG⋅MP and hence MD is tangent to the circumcircle of △DGP. Therefore ∠DGP=∠EDP. Let Γ′ be the circumcircle of △BDE. If B=P, then, since ∠BGD=∠BDE, the tangent lines of Γ′ and Γ at B should coincide, that is Γ′ is tangent to Γ from inside. Let B=P. If P lies in the same side of the line BC as G, then we have ∠EDP+∠ABP=180∘ because ∠DGP+∠ABP=180∘. That is, the quadrilateral BPDE is cyclic, and hence P is on the intersection of Γ′ with Γ. Otherwise, ∠EDP=∠DGP=∠AGP=∠ABP=∠EBP. Therefore the quadrilateral PBDE is cyclic, and hence P again is on the intersection of Γ′ with Γ. Similarly, if LH meets Γ at Q, we either have Q=B, in which case Γ′ is tangent to Γ from inside, or Q=B. In the latter case, Q is on the intersection of Γ′ with Γ. In either case, we have P=Q.
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