Let ABC be a triangle, and let BE,CF be the altitudes. Let ℓ be a line passing through A. Suppose ℓ intersect BE at P, and ℓ intersect CF at Q. Prove that: i) If ℓ is the A-median, then circles (APF) and (AQE) are tangent. ii) If ℓ is the inner A-angle bisector, suppose (APF) intersect (AQE) again at R, then AR is perpendicular to ℓ.
Problem
Source: Own. IMO 2022 Malaysian Training Camp 2
Tags: geometry
13.03.2022 16:29
11.08.2023 23:55
A solution for the first problem: Let S=AB∩(FPQ), T=AC∩(EPQ), M the midpoint of BC and N=AM∩ST. Then we have that: AF⋅AS=AP⋅AQ=AE⋅AT and hence SFET is cyclic. So: ∠AST=∠AEF=∠ABC=>ST∥BC=>N is the midpoint of ST=>NS=NT(1). Also from the cyclic quadrilaterals we get that: ∠SPQ=∠SFQ=∠BFC=90=∠BEC=∠PET=∠MQT, hence SP∥QT(2), as they are both perpendicular to AM. From (1) and (2) we get that SPTQ is a parallelogram. Using the parallelogram and the cyclic quadrilaterals: ∠AFP=∠PQS=∠QPT=∠QET. Now let X be a point such that XC<XB and AX is tangent to (AQE). Then we have that: ∠AFP=∠QET=∠QAE+∠AQE=∠QAE+∠XAE=∠XAQ=∠XAP, hence AX is also tangent to (APF), which proves the desired tangency.
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12.08.2023 12:54
For the second problem: Let O1 be the center of (AFP), O2 be the center of (AQE) and H=BE∩CF . By radical axis we have that O1O2⊥AR so it suffices to prove that O1O2∥ℓ. Since O1 is the circumcenter of AFP we conclude that: ∠FO1P=2∠FAP=2∠BAP=2∠A/2=∠A=180−∠BHC=180−∠FHP, hence FHPO1 is cyclic. Hence, using that FO1=PO1, we have that O1 is the midpoint of the arc FP (that does not contain H) in (FHP). So HO1 is angle bisector of ∠FHP, and hence angle bisector of ∠BHC, which is obviously parallel to ℓ. Hence HO1∥ℓ and analogously we have that HO2∥ℓ. So we have that O1O2∥ℓ, as needed.
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17.08.2023 09:27
For i) Let Mbe the mid-point of BC,suppose X,Y∈AM satisfy ∠BXM=90∘−∠ACB,∠CYM=90∘−∠ABC respectively, T be the H-Humpty point of △ABC Consider an inversion IMMB2 So P↦X,Q↦Y,A↦T,F↦F,E↦E Hence ⊙(APF)↦⊙(XFT),⊙(AQE)↦⊙(YET) We need to prove that ⊙(XFT) is tangent to ⊙(YET)⇒∠YET+∠XFT=180∘ Note that H,B,C,T are cyclic Hence ∠HTC=180∘−∠HBC=180∘−∠BXM=∠AXB Note that ∠TBM=∠BAM Hence ∠THC=∠BAX Which means that △BAX∼△CTH Similarly △CYA∼△BTH Consider that MP⋅MX=MB2=ME2 Hence ∠EXM=∠BEM=∠EBM Which means that ∠AXE=180∘−∠TXE=180∘−∠EBC=180∘−∠EAH=∠HTE So △EAX∼△EHT Similarly △FYA∼△FTH Since XBYC=AX⋅TCHTAY⋅BTHT=AXAY⋅TCTB=AXHTAYHT⋅ACAB=AEAF⋅HFHE⋅ACAB=HFHE Which means that △XBF∼△YCE So ∠YET+∠XFT=∠AET+180∘−∠YEC+∠AFT−180∘+∠TFB=∠AET+∠AFT Note that A,F,H,T,E are cyclic So ∠YET+∠XFT=∠AET+∠AFT=180∘ Which means that ⊙(APF) and ⊙(AQE) are tangent
17.08.2023 09:33
For ii) Let O1,O2 be the circumcenter of △AFP and △AEQ respectively Note that AR is the root axis of ⊙(AFP) and ⊙(AEQ) so we need to prove that O1O2//AQ Consider that ∠FO1P=∠BAC=180∘−∠FHP Which means that F,O1,P,H are cyclic Hence ∠O1HF=∠O1PF=90∘−12∠BAC Similarly ∠O2HQ=180∘−∠O2EQ=90∘+12∠BAC So ∠O1HF+∠O2HF=180∘ Which means that O1,O2,H are collinear Since ∠O1HF=∠O1PF=90∘−∠FAQ=∠AQF Hence O1O2//AQ Which means that AR⊥l