Maths Olympiad Prep

Track / Stage 8 / 82 of 180 #1782 of 1964

Problem 1782

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 \ell be a line passing through AA. Suppose \ell intersect BEBE at PP, and \ell intersect CFCF at QQ. Prove that:

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

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

Official solution

### Part (i)

1. Define Points and Setup:
Let M M be the midpoint of BC BC . Suppose X,YAM X, Y \in AM satisfy BXM=90ACB \angle BXM = 90^\circ - \angle ACB and CYM=90ABC \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:
- PX P \mapsto X
- QY Q \mapsto Y
- AT A \mapsto T
- FF F \mapsto F
- EE 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=180HBC=180BXM=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 BAXCTH\triangle BAX \sim \triangle CTH. Similarly, CYABTH\triangle CYA \sim \triangle BTH.

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

7. Proportionality:
Since XBYC=AXTCHTAYBTHT=AXAYTCTB=AXHTAYHTACAB=AEAFHFHEACAB=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 XBFYCE\triangle XBF \sim \triangle YCE.

8. Angle Sum:
So, YET+XFT=AET+180YEC+AFT180+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 \ell be the inner AA-angle bisector. Suppose (APF)(APF) intersects (AQE)(AQE) again at RR.

2. Angle Bisector Theorem:
Since \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 \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 \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 \ell.

\blacksquare

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.