Maths Olympiad Prep

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Geometry Difficulty 6.5 National Olympiad Prove it Taiwan

Let II and IAI_A be the incenter and the AA-excenter of an acute-angled triangle ABCABC, with AB<ACAB < AC. Let the incircle meet BCBC at DD. The line ADAD meets BIABI_A and CIACI_A at EE and FF, respectively. Prove that the circumcircles of triangles AIDAID and IAEFIAEF are tangent to each other.

Solutions — 2

Solution 1

Let (p,q)\angle(p, q) denote the directed angle between lines pp and qq.
The points B,C,IB, C, I, and IAI_A lie on the circle Γ\Gamma with diameter IIAII_A. Let ω\omega and Ω\Omega denote the circles (IAEF)(IAEF) and (AID)(AID), respectively. Let TT be the second intersection point of ω\omega and Γ\Gamma. Then TT is the Miquel point of the complete quadrilateral formed by the lines BC,BIA,CIABC, BI_A, CI_A, and DEFDEF, so TT also lies on circle (BDE)(BDE) (as well as on circle (CDF)(CDF)). We claim that TT is the desired tangency point of ω\omega and Ω\Omega.
In order to show that TT lies on Ω\Omega, use cyclic quadrilaterals BDETBDET and BIIATBII_AT to write
(DT,DA)=(DT,DE)=(BT,BE)=(BT,BIA)=(IT,IIA)=(IT,IA). \angle(DT, DA) = \angle(DT, DE) = \angle(BT, BE) = \angle(BT, BI_A) = \angle(IT, II_A) = \angle(IT, IA).
To show that ω\omega and Ω\Omega are tangent at TT, let \ell be the tangent to ω\omega at TT, so that
(TIA,)=(EIA,ET). \angle(TI_A, \ell) = \angle(EI_A, ET).
Using circles (BDET)(BDET) and (BICIA)(BICI_A), we get
(EIA,ET)=(EB,ET)=(DB,DT). \angle(EI_A, ET) = \angle(EB, ET) = \angle(DB, DT).
Therefore,
(TI,)=90+(TIA,)=90+(DB,DT)=(DI,DT), \angle(TI, \ell) = 90^\circ + \angle(TI_A, \ell) = 90^\circ + \angle(DB, DT) = \angle(DI, DT),
which shows that \ell is tangent to Ω\Omega at TT.

Solution 2

We use the notation of circles Γ,ω\Gamma, \omega, and Ω\Omega as in the previous solution.
Let LL be the point opposite to II in circle Ω\Omega. Then IAL=IDL=90\angle IAL = \angle IDL = 90^\circ, which means that LL is the foot of the external bisector of A\angle A in triangle ABCABC. Let LILI cross Γ\Gamma again at MM.
Let TT be the foot of the perpendicular from II onto IALI_A L. Then TT is the second intersection point of Ω\Omega and Γ\Gamma. We will show that TT is the desired tangency point.

(LT,LM)=(AT,AI)and(MT,ML)=(MT,MI)=(IAT,IAI), \angle(LT, LM) = \angle(AT, AI) \quad \text{and} \quad \angle(MT, ML) = \angle(MT, MI) = \angle(I_A T, I_A I),
which shows that triangles TMLTML and TIAATIA_A are similar and equioriented. So there exists a rotational homothety τ\tau mapping TMLTML to TIAATIA_A.
Since (ML,LD)=(AI,AD)\angle(ML, LD) = \angle(AI, AD), we get τ(BC)=AD\tau(BC) = AD. Next, since
(MB,ML)=(MB,MI)=(IAB,IAI)=(IAE,IAA), \angle(MB, ML) = \angle(MB, MI) = \angle(I_A B, I_A I) = \angle(I_A E, I_A A),
we get τ(B)=E\tau(B) = E. Similarly τ(C)=F\tau(C) = F. Since the points M,B,CM, B, C, and TT are concyclic, so are their τ\tau-images, which means that TT lies on ω=τ(Γ)\omega = \tau(\Gamma).
Finally, since τ(L)=A\tau(L) = A and τ(B)=E\tau(B) = E, triangles ATLATL and ETBETB are similar so that
(AT,AL)=(ET,EB)=(EIA,ET). \angle(AT, AL) = \angle(ET, EB) = \angle(EI_A, ET).
This means that the tangents to Ω\Omega and ω\omega at TT make the same angle with the line IATLI_A TL, so the circles are indeed tangent at TT.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.