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Geometry Difficulty 7.4 National olympiad, round 2 Prove it

In an acute triangle ABC ABC segments BE BE and CF CF are altitudes. Two circles passing through the point A A and F F and tangent to the line BC BC at the points P P and Q Q so that B B lies between C C and Q Q. Prove that lines PE PE and QF QF intersect on the circumcircle of triangle AEF AEF.

Proposed by Davood Vakili, Iran

Solution

1. Define the orthocenter and projections: Let H H be the orthocenter of ABC \triangle ABC , and let D D be the projection of A A onto BC BC .

2. Consider the inversion: Consider the inversion through pole H H with power k=HAHD=HEHB=HCHF k = \overline{HA} \cdot \overline{HD} = \overline{HE} \cdot \overline{HB} = \overline{HC} \cdot \overline{HF} . This inversion maps P P to P P' and Q Q to Q Q' .

3. **Mapping of line BC BC **: The inversion I(H,k) \mathcal{I}(H, k) maps the line BC BC to the circumcircle of AEF \triangle AEF . Therefore, P P' and Q Q' both lie on the circumcircle of AEF \triangle AEF .

4. Orthocenter properties: Since H H is the orthocenter of ABC \triangle ABC , it will also be the orthocenter of AQP \triangle AQP . Thus, Q Q' and P P' are the projections of P P and Q Q onto AQ AQ and AP AP , respectively.

5. Radical axis: The line AF AF is the radical axis of the two circles passing through A A and F F and tangent to BC BC at P P and Q Q , respectively. Therefore, AF AF will go to the midpoint B B of PQ PQ .

6. Equal power of point: We have BP2=BQ2=BFBA=BDBC BP^2 = BQ^2 = \overline{BF} \cdot \overline{BA} = \overline{BD} \cdot \overline{BC} . If C C' is the intersection of PQ P'Q' with PQ PQ , then (QPDC) (QPDC') is harmonic.

7. MacLaurin identity: By the MacLaurin identity, BQ2=BP2=BDBC BQ^2 = BP^2 = \overline{BD} \cdot \overline{BC'} , which implies C C must coincide with C C' . Therefore, P P' , Q Q' , and C C are collinear.

8. Harmonic division: Since (CDPQ) (CDPQ) is harmonic, we have CACE=CPCQ=CDCB \overline{CA} \cdot \overline{CE} = \overline{CP} \cdot \overline{CQ} = \overline{CD} \cdot \overline{CB} . This implies E(AQP) E \in (AQP) . Similarly, F(HPQ) F \in (HPQ) .

9. Angle chasing: Finally, we have (FQ,EP)=(FQ,QP)+(QP,EP)=(HF,HP)+(AQ,AE)=(AF,AP)+(AP,AE)=(AF,AE) \angle (FQ, EP) = \angle (FQ, QP) + \angle (QP, EP) = \angle (HF, HP') + \angle (AQ, AE) = \angle (AF, AP') + \angle (AP', AE) = \angle (AF, AE) .

10. Intersection point: Thus, if W W is the intersection of EP EP with FQ FQ , then W(AEF) W \in (AEF) .

\blacksquare

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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.