Maths Olympiad Prep

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Geometry Difficulty 6.6 National olympiad Prove it

Let Γ\Gamma be a circle of center OO, and δ\delta be a line in the plane of Γ\Gamma, not intersecting it. Denote by AA the foot of the perpendicular from OO onto δ\delta, and let MM be a (variable) point on Γ\Gamma. Denote by γ\gamma the circle of diameter AMA M, by XX the (other than MM) intersection point of γ\gamma and Γ\Gamma, and by YY the (other than AA) intersection point of γ\gamma and δ\delta. Prove that the line XYX Y passes through a fixed point.

Solution

Consider the line ρ\rho tangent to γ\gamma at AA, and take the points {K}=AMXY,{L}=\{K\}=A M \cap X Y,\{L\}= ρXM\rho \cap X M, and {F}=OAXY\{F\}=O A \cap X Y.

(Remark: Moving MM into its reflection with respect to the line OAO A will move XYX Y into its reflection with respect to OAO A. These old and the new XYX Y meet on OAO A, hence it should be clear that the fixed point must be FF.)

Since \varangleLMA=\varangleFYA\varangle L M A=\varangle F Y A and \varangleYAF=\varangleLAM=90\varangle Y A F=\varangle L A M=90^{\circ}, it follows that triangles FAYF A Y and LAML A M are similar, therefore \varangleAFY=\varangleALM\varangle A F Y=\varangle A L M, hence the quadrilateral ALXFA L X F is cyclic. But then \varangleAFL=\varangleAXL=90\varangle A F L=\varangle A X L=90^{\circ}, so LFAFL F \perp A F, hence LFδL F \| \delta.

Now, ρ\rho is the radical axis of circles γ\gamma and AA (consider AA as a circle of center AA and radius 0 ), while XMX M is the radical axis of circles γ\gamma and Γ\Gamma, so LL is the radical center of the three circles, which means that LL lies on the radical axis of circles Γ\Gamma and AA. From LFOAL F \perp O A, where OAO A is the line of the centers of the circles AA and Γ\Gamma, and FXYF \in X Y, it follows that FF is (the) fixed point of XYX Y.

(The degenerate two cases when MOAM \in O A, where XMX \equiv M and YAY \equiv A, also trivially satisfy the conclusion, as then FAM)F \in A M).

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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.