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Geometry Difficulty 8.6 Shortlist Prove it IMO

Let ABCABC be an acute-angled triangle with AB<ACAB < AC. Denote its circumcircle by Ω\Omega and denote the midpoint of arcCAB\operatorname{arc} CAB by SS. Let the perpendicular from AA to BCBC meet BSBS and Ω\Omega at DD and EAE \neq A respectively. Let the line through DD parallel to BCBC meet line BEBE at LL and denote the circumcircle of triangle BDLBDL by ω\omega. Let ω\omega meet Ω\Omega again at PBP \neq B.
Prove that the line tangent to ω\omega at PP, and line BSBS intersect on the internal bisector of BAC\angle BAC.

Solutions — 3

Solution 1

Let SS' be the midpoint of arcBC\operatorname{arc} BC of Ω\Omega, diametrically opposite to SS so SSSS' is a diameter in Ω\Omega and ASAS' is the angle bisector of BAC\angle BAC. Let the tangent of ω\omega at PP meet Ω\Omega again at QPQ \neq P, then we have SQS=90\angle SQS' = 90^\circ.
We will show that triangles APDAPD and SQSS'QS are similar and their corresponding sides are parallel. Then it will follow that the lines connecting the corresponding vertices, namely line ASAS', that is the angle bisector of BAC\angle BAC, line PQPQ, that is the tangent to ω\omega at PP, and DSDS are concurrent. Note that the sides ADAD and SSS'S have opposite directions, so the three lines cannot be parallel.

Figure 1

First we show that APDPAP \perp DP. Indeed, from cyclic quadrilaterals APBEAPBE and DPLBDPLB we can see that
PAD=PAE=180EBP=PBL=PDL=90ADP. \angle PAD = \angle PAE = 180^\circ - \angle EBP = \angle PBL = \angle PDL = 90^\circ - \angle ADP.
Then, in triangle APDAPD we have DPA=180PADADP=90\angle DPA = 180^\circ - \angle PAD - \angle ADP = 90^\circ.

Now we can see that:
- Both lines ADEADE and SSSS' are perpendicular to BCBC, so ADSSAD \parallel S'S.
- Line PQPQ is tangent to circle ω\omega at PP so DPQ=DBP=SBP=SQP\angle DPQ = \angle DBP = \angle SBP = \angle SQP; it follows that PDQSPD \parallel QS.
- Finally, since APPDQSSQAP \perp PD \parallel QS \perp S'Q, we have APSQAP \parallel S'Q as well.

Hence the corresponding sides of triangles APDAPD and SQSS'QS are parallel completing the solution.

Solution 2

Again, let SS' be the midpoint of arc BCBC, diametrically opposite to SS, so AESSAES'S is an isosceles trapezoid, and SBS=SPS=90\angle S'BS = \angle S'PS = 90^\circ. Let lines AEAE and PSPS' meet at TT and let APAP and SBS'B meet at point MM.
We will need that points L,P,SL, P, S are collinear, and points TT and MM lie on circle ω\omega.
- From LPB=LDB=90BDE=90BSS=SSB=180BPS\angle LPB = \angle LDB = 90^\circ - \angle BDE = 90^\circ - \angle BSS' = \angle SS'B = 180^\circ - \angle BPS we get LPB+BPS=180\angle LPB + \angle BPS = 180^\circ, so L,PL, P and SS are indeed collinear.
- Since SSSS' is a diameter in Ω\Omega, lines LPSLPS and PTSPTS' are perpendicular. We also have LDBCAELD \parallel BC \perp AE hence LDT=LPT=90\angle LDT = \angle LPT = 90^\circ and therefore TωT \in \omega.
- By LPM=SPA=SEA=EAS=EBS=LBM\angle LPM = \angle SPA = \angle SEA = \angle EAS' = \angle EBS' = \angle LBM, point MM is concyclic with B,P,LB, P, L so MωM \in \omega.

Figure 2

Now let XX be the intersection of line BDSBDS with the tangent of ω\omega at PP and apply Pascal's theorem to the degenerate cyclic hexagon PPMBDTPPMBDT. This gives points PPBD=XPP \cap BD = X, PMDT=APM \cap DT = A and MBTP=SMB \cap TP = S' are collinear so XX lies on line ASAS', that is the bisector of BAC\angle BAC.

Solution 3

Let AA' and SS' be the points of Ω\Omega diametrically opposite to AA and SS respectively. It is well-known that EE and AA' are reflections with respect to SSSS' so ASAS' is the angle bisector of EAA\angle EAA'. Define point TT to be the intersection of AEAE and PSPS'. As in the previous two solutions, we have: DPA=90\angle DPA = 90^\circ so PDPD passes through AA'; points L,P,SL, P, S are collinear; and TωT \in \omega.
Let lines ASAS' and PDAPDA' meet at RR. From the angles of triangles PRSPRS' and PTEPTE we get
ARP=ASP+SPA=AEP+EPS=ATP \angle ARP = \angle AS'P + \angle S'PA' = \angle AEP + \angle EPS' = \angle ATP
so points A,P,T,RA, P, T, R are concyclic. Denote their circle by γ\gamma. Due to RPA=DPA=90\angle RPA = \angle DPA = 90^\circ, segment ARAR is a diameter in γ\gamma.

Figure 3

We claim that circles ω\omega and γ\gamma are perpendicular. Let line LPSLPS meet γ\gamma again at UPU \neq P, and consider triangles PLTPLT and PTUPTU. By LPT=TPU=90\angle LPT = \angle TPU = 90^\circ and
PTL=PBL=180EBP=PAE=PAT=PUT, \angle PTL = \angle PBL = 180^\circ - \angle EBP = \angle PAE = \angle PAT = \angle PUT,
triangles PLTPLT and PTUPTU are similar. It follows that the spiral similarity that takes PLTPLT to PTUPTU, maps ω\omega to γ\gamma and the angle of this similarity is 9090^\circ, so circles ω\omega and γ\gamma are indeed perpendicular.

Finally, let lines BDSBDS and ARSARS' meet at XX. We claim that XX bisects ARAR, so point XX is the centre of γ\gamma and, as ω\omega and γ\gamma are perpendicular, PXPX is tangent to ω\omega.
Let tt be the tangent of ω\omega at DD. From (DT,t)=TPD=SPA=EAS\angle(DT, t) = \angle TPD = \angle S'PA' = \angle EAS' it can be seen that tASt \parallel AS'. Let II be the common point at infinity of tt and ASAS'. Moreover, let lines LPSLPS and ADTEADTE meet at VV. By projecting line ASAS' to circle ω\omega through DD, then projecting ω\omega to line AEAE through LL, finally projecting AEAE to Ω\Omega through PP, we find
AXRX=(A,R;X,I)=D(T,P;B,D)=L(T,V;E,D)=P(S,S;E,A)=1, \frac{AX}{RX} = (A, R; X, I) \stackrel{D}{=} (T, P; B, D) \stackrel{L}{=} (T, V; E, D) \stackrel{P}{=} (S', S; E, A') = -1,
so XX is the midpoint of ARAR.

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