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Geometry Difficulty 8.2 Shortlist Prove it IMO

Let ABCA B C be an acute triangle with AB>ACA B > A C, and let Γ\Gamma be its circumcircle. Let HH, MM, and FF be the orthocenter of the triangle, the midpoint of BCB C, and the foot of the altitude from AA, respectively. Let QQ and KK be the two points on Γ\Gamma that satisfy ∠AQH=90∘\angle A Q H = 90^{\circ} and ∠QKH=90∘\angle Q K H = 90^{\circ}. Prove that the circumcircles of the triangles KQHK Q H and KFMK F M are tangent to each other.

Solutions — 2

Solution 1

Consider any point TT such that TKT K is tangent to the circle KQHK Q H at KK with QQ and TT lying on different sides of KHK H (see Figure 1). Then ∠HKT=∠HQK\angle H K T = \angle H Q K and we are to prove that ∠MKT=∠CFK\angle M K T = \angle C F K. Thus it remains to show that ∠HQK=∠CFK+∠HKM\angle H Q K = \angle C F K + \angle H K M. Due to ∠HQK=90∘−∠Q′HA′\angle H Q K = 90^{\circ} - \angle Q' H A', and ∠CFK=90∘−∠KFA\angle C F K = 90^{\circ} - \angle K F A, this means the same as ∠Q′HA′=∠KFA−∠HKM\angle Q' H A' = \angle K F A - \angle H K M. Now, since the triangles KHEK H E and AHQ′A H Q' are similar with FF and JJ being the midpoints of corresponding sides, we have ∠KFA=∠HJA\angle K F A = \angle H J A, and analogously one may obtain ∠HKM=∠JQH\angle H K M = \angle J Q H. Thereby our task is reduced to verifying
∠Q′HA′=∠HJA−∠JQH. \angle Q' H A' = \angle H J A - \angle J Q H .
Figure 1
Figure 1
Figure 2
Figure 2
To avoid confusion, let us draw a new picture at this moment (see Figure 2). Owing to ∠Q′HA′=∠JQH+∠HJQ\angle Q' H A' = \angle J Q H + \angle H J Q and ∠HJA=∠QJA+∠HJQ\angle H J A = \angle Q J A + \angle H J Q, we just have to show that 2∠JQH=∠QJA2 \angle J Q H = \angle Q J A. To this end, it suffices to remark that AQA′Q′A Q A' Q' is a rectangle and that JJ, being defined to be the midpoint of HQ′H Q', has to lie on the mid parallel of QA′Q A' and Q′AQ' A.

Solution 2

We define the points A′A' and EE and prove that the ray MHM H passes through QQ in the same way as in the first solution. Notice that the points A′A' and EE can play analogous roles to the points QQ and KK, respectively: point A′A' is the second intersection of the line MHM H with Γ\Gamma, and EE is the point on Γ\Gamma with the property ∠HEA′=90∘\angle H E A' = 90^{\circ} (see Figure 3).

In the circles KQHK Q H and EA′HE A' H, the line segments HQH Q and HA′H A' are diameters, respectively; so, these circles have a common tangent tt at HH, perpendicular to MHM H. Let RR be the radical center of the circles ABC,KQHA B C, K Q H and EA′HE A' H. Their pairwise radical axes are the lines QKQ K, A′EA' E and the line tt; they all pass through RR. Let SS be the midpoint of HRH R; by ∠QKH=∠HEA′=90∘\angle Q K H = \angle H E A' = 90^{\circ}, the quadrilateral HERKH E R K is cyclic and its circumcenter is SS; hence we have SK=SE=SHS K = S E = S H. The line BCB C, being the perpendicular bisector of HEH E, passes through SS.
The circle HMFH M F also is tangent to tt at HH; from the power of SS with respect to the circle HMFH M F we have
SM⋅SF=SH2=SK2. S M \cdot S F = S H^{2} = S K^{2} .
So, the power of SS with respect to the circles KQHK Q H and KFMK F M is SK2S K^{2}. Therefore, the line segment SKS K is tangent to both circles at KK.

Figure 3
Figure 3

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