Maths Olympiad Prep

Track / Stage 8 / 102 of 180 #1802 of 1964

Problem 1802

IMO Shortlist mid-range; USAMO P2/P5
Geometry Difficulty 8.2 Prove it

Let ABCABC be a triangle, and let BE,CFBE, CF be the altitudes. Let ℓ\ell be a line passing through AA. Suppose ℓ\ell intersect BEBE at PP, and ℓ\ell intersect CFCF at QQ. Prove that:

i) If ℓ\ell is the AA-median, then circles (APF)(APF) and (AQE)(AQE) are tangent.

ii) If ℓ\ell is the inner AA-angle bisector, suppose (APF)(APF) intersect (AQE)(AQE) again at RR, then ARAR is perpendicular to ℓ\ell.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Next problem →

Official solution

### Part (i)

1. Define Points and Setup:
Let M M be the midpoint of BC BC . Suppose X,Y∈AM X, Y \in AM satisfy ∠BXM=90∘−∠ACB \angle BXM = 90^\circ - \angle ACB and ∠CYM=90∘−∠ABC \angle CYM = 90^\circ - \angle ABC respectively. Let T T be the H H -Humpty point of △ABC \triangle ABC .

2. Inversion:
Consider an inversion IMB2M\mathcal{I}^{M}_{MB^{2}}. Under this inversion:
- P↦X P \mapsto X
- Q↦Y Q \mapsto Y
- A↦T A \mapsto T
- F↦F F \mapsto F
- E↦E E \mapsto E

Hence, ⊙(APF)↦⊙(XFT)\odot(APF) \mapsto \odot(XFT) and ⊙(AQE)↦⊙(YET)\odot(AQE) \mapsto \odot(YET).

3. Tangency Condition:
We need to prove that ⊙(XFT)\odot(XFT) is tangent to ⊙(YET)\odot(YET), which implies ∠YET+∠XFT=180∘\angle YET + \angle XFT = 180^\circ.

4. Cyclic Quadrilateral:
Note that H,B,C,T H, B, C, T are cyclic. Hence, ∠HTC=180∘−∠HBC=180∘−∠BXM=∠AXB\angle HTC = 180^\circ - \angle HBC = 180^\circ - \angle BXM = \angle AXB.

5. Angle Relationships:
Note that ∠TBM=∠BAM\angle TBM = \angle BAM. Hence, ∠THC=∠BAX\angle THC = \angle BAX, which means that △BAX∼△CTH\triangle BAX \sim \triangle CTH. Similarly, △CYA∼△BTH\triangle CYA \sim \triangle BTH.

6. Inversion Properties:
Consider that MP⋅MX=MB2=ME2 MP \cdot MX = MB^2 = ME^2 . Hence, ∠EXM=∠BEM=∠EBM\angle EXM = \angle BEM = \angle EBM, which means that ∠AXE=180∘−∠TXE=180∘−∠EBC=180∘−∠EAH=∠HTE\angle AXE = 180^\circ - \angle TXE = 180^\circ - \angle EBC = 180^\circ - \angle EAH = \angle HTE. So, △EAX∼△EHT\triangle EAX \sim \triangle EHT. Similarly, △FYA∼△FTH\triangle FYA \sim \triangle FTH.

7. Proportionality:
Since XBYC=AX⋅TCHTAY⋅BTHT=AXAY⋅TCTB=AXHTAYHT⋅ACAB=AEAF⋅HFHE⋅ACAB=HFHE\frac{XB}{YC} = \frac{\frac{AX \cdot TC}{HT}}{\frac{AY \cdot BT}{HT}} = \frac{AX}{AY} \cdot \frac{TC}{TB} = \frac{\frac{AX}{HT}}{\frac{AY}{HT}} \cdot \frac{AC}{AB} = \frac{AE}{AF} \cdot \frac{HF}{HE} \cdot \frac{AC}{AB} = \frac{HF}{HE}, which means that △XBF∼△YCE\triangle XBF \sim \triangle YCE.

8. Angle Sum:
So, ∠YET+∠XFT=∠AET+180∘−∠YEC+∠AFT−180∘+∠TFB=∠AET+∠AFT\angle YET + \angle XFT = \angle AET + 180^\circ - \angle YEC + \angle AFT - 180^\circ + \angle TFB = \angle AET + \angle AFT.

9. Cyclic Points:
Note that A,F,H,T,E A, F, H, T, E are cyclic. So, ∠YET+∠XFT=∠AET+∠AFT=180∘\angle YET + \angle XFT = \angle AET + \angle AFT = 180^\circ, which means that ⊙(APF)\odot(APF) and ⊙(AQE)\odot(AQE) are tangent.

■\blacksquare

### Part (ii)

1. Define Points and Setup:
Let ℓ\ell be the inner AA-angle bisector. Suppose (APF)(APF) intersects (AQE)(AQE) again at RR.

2. Angle Bisector Theorem:
Since ℓ\ell is the inner AA-angle bisector, it divides ∠BAC\angle BAC into two equal angles.

3. Intersection Point:
Let RR be the intersection point of (APF)(APF) and (AQE)(AQE) other than AA.

4. Perpendicularity:
We need to show that ARAR is perpendicular to ℓ\ell.

5. Cyclic Quadrilateral:
Since RR lies on both circles (APF)(APF) and (AQE)(AQE), we have ∠ARF=∠APF\angle ARF = \angle APF and ∠ARE=∠AQE\angle ARE = \angle AQE.

6. Angle Sum:
Since ℓ\ell is the angle bisector, ∠PAF=∠QAE\angle PAF = \angle QAE. Therefore, ∠ARF+∠ARE=∠APF+∠AQE=180∘\angle ARF + \angle ARE = \angle APF + \angle AQE = 180^\circ.

7. Perpendicularity Conclusion:
Since ∠ARF+∠ARE=180∘\angle ARF + \angle ARE = 180^\circ, it implies that ARAR is perpendicular to ℓ\ell.

■\blacksquare

Source: NuminaMath-1.5, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.