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Geometry Difficulty 7.2 National olympiad, round 2 Prove it

Let ABCDA B C D be a quadrilateral inscribed in a circle Ω\Omega. Let the tangent to Ω\Omega at DD intersect the rays BAB A and BCB C at points EE and FF, respectively. A point TT is chosen inside the triangle ABCA B C so that TECDT E \| C D and TFADT F \| A D. Let KDK \neq D be a point on the segment DFD F such that TD=TKT D = T K. Prove that the lines AC,DTA C, D T and BKB K intersect at one point.

Solution

Let the segments TET E and TFT F cross ACA C at PP and QQ, respectively. Since PECDP E \| C D and EDE D is tangent to the circumcircle of ABCDA B C D, we have
EPA=DCA=EDA, \angle E P A=\angle D C A=\angle E D A,
and so the points A,P,DA, P, D, and EE lie on some circle α\alpha. Similarly, the points C,Q,DC, Q, D, and FF lie on some circle γ\gamma. We now want to prove that the line DTD T is tangent to both α\alpha and γ\gamma at DD. Indeed, since FCD+EAD=180\angle F C D + \angle E A D = 180^{\circ}, the circles α\alpha and γ\gamma are tangent to each other at DD. To prove that TT lies on their common tangent line at DD (i.e., on their radical axis), it suffices to check that TPTE=TQTFT P \cdot T E = T Q \cdot T F, or that the quadrilateral PEFQP E F Q is cyclic. This fact follows from
QFE=ADE=APE. \angle Q F E = \angle A D E = \angle A P E.
Since TD=TKT D = T K, we have TKD=TDK\angle T K D = \angle T D K. Next, as TDT D and DED E are tangent to α\alpha and Ω\Omega, respectively, we obtain
TKD=TDK=EAD=BDE, \angle T K D = \angle T D K = \angle E A D = \angle B D E,
which implies TKBDT K \| B D. Next we prove that the five points T,P,Q,DT, P, Q, D, and KK lie on some circle τ\tau. Indeed, since TDT D is tangent to the circle α\alpha we have
EPD=TDF=TKD, \angle E P D = \angle T D F = \angle T K D,
which means that the point PP lies on the circle (TDK). Similarly, we have Q(TDK)Q \in (T D K). Finally, we prove that PKBCP K \| B C. Indeed, using the circles τ\tau and γ\gamma we conclude that
PKD=PQD=DFC, \angle P K D = \angle P Q D = \angle D F C,
which means that PKBCP K \| B C. Triangles TPKT P K and DCBD C B have pairwise parallel sides, which implies the fact that TD,PCT D, P C and KBK B are concurrent, as desired. !

Comment 1. There are several variations of the above solution. E.g., after finding circles α\alpha and γ\gamma, one can notice that there exists a homothety hh mapping the triangle TPQT P Q to the triangle DCAD C A; the centre of that homothety is Y=ACTDY = A C \cap T D. Since
DPE=DAE=DCB=DQT, \angle D P E = \angle D A E = \angle D C B = \angle D Q T,
the quadrilateral TPDQT P D Q is inscribed in some circle τ\tau. We have h(τ)=Ωh(\tau) = \Omega, so the point D=h(D)D^{*} = h(D) lies on Ω\Omega.
Finally, by
DCD=TPD=BAD, \angle D C D^{*} = \angle T P D = \angle B A D,
the points BB and DD^{*} are symmetric with respect to the diameter of Ω\Omega passing through DD. This yields DB=DDD B = D D^{*} and BDEFB D^{*} \| E F, so h(K)=Bh(K) = B, and BKB K passes through YY.

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