1. Define the orthocenter and projections: Let H be the orthocenter of △ABC, and let D be the projection of A onto BC.
2. Consider the inversion: Consider the inversion through pole H with power k=HA⋅HD=HE⋅HB=HC⋅HF. This inversion maps P to P′ and Q to Q′.
3. **Mapping of line BC**: The inversion I(H,k) maps the line BC to the circumcircle of △AEF. Therefore, P′ and Q′ both lie on the circumcircle of △AEF.
4. Orthocenter properties: Since H is the orthocenter of △ABC, it will also be the orthocenter of △AQP. Thus, Q′ and P′ are the projections of P and Q onto AQ and AP, respectively.
5. Radical axis: The line AF is the radical axis of the two circles passing through A and F and tangent to BC at P and Q, respectively. Therefore, AF will go to the midpoint B of PQ.
6. Equal power of point: We have BP2=BQ2=BF⋅BA=BD⋅BC. If C′ is the intersection of P′Q′ with PQ, then (QPDC′) is harmonic.
7. MacLaurin identity: By the MacLaurin identity, BQ2=BP2=BD⋅BC′, which implies C must coincide with C′. Therefore, P′, Q′, and C are collinear.
8. Harmonic division: Since (CDPQ) is harmonic, we have CA⋅CE=CP⋅CQ=CD⋅CB. This implies E∈(AQP). Similarly, F∈(HPQ).
9. Angle chasing: Finally, we have ∠(FQ,EP)=∠(FQ,QP)+∠(QP,EP)=∠(HF,HP′)+∠(AQ,AE)=∠(AF,AP′)+∠(AP′,AE)=∠(AF,AE).
10. Intersection point: Thus, if W is the intersection of EP with FQ, then W∈(AEF).
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