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Geometry Difficulty 6.1 National olympiad Prove it Asia Pacific Mathematics Olympiad (APMO)

Let Γ\Gamma be the circumcircle of ABC\triangle A B C. Let DD be a point on the side BCB C. The tangent to Γ\Gamma at AA intersects the parallel line to BAB A through DD at point EE. The segment CEC E intersects Γ\Gamma again at FF. Suppose B,D,F,EB, D, F, E are concyclic. Prove that AC,BF,DEA C, B F, D E are concurrent.

Solutions — 2

Solution 1

From the conditions, we have
Figure 1
CBA=180EDB=180EFB=180EFAAFB=180CBAACB=BAC. \begin{aligned} \angle C B A & =180^\circ-\angle E D B=180^\circ-\angle E F B \\ & =180^\circ-\angle E F A-\angle A F B \\ & =180^\circ-\angle C B A-\angle A C B=\angle B A C . \end{aligned}
Let PP be the intersection of ACA C and BFB F. Then we have
PAE=CBA=BAC=BFC. \angle P A E=\angle C B A=\angle B A C=\angle B F C .
This implies A,P,F,EA, P, F, E are concyclic. It follows that
FPE=FAE=FBA, \angle F P E=\angle F A E=\angle F B A,
and hence ABA B and EPE P are parallel. So E,P,DE, P, D are collinear, and the result follows.

Solution 2

Let EE^{\prime} be any point on the extension of EAE A. From AED=EAB=ACD\angle A E D=\angle E^{\prime} A B=\angle A C D, points A,D,C,EA, D, C, E are concyclic.
Figure 2
Let PP be the intersection of BFB F and DED E. From AFP=ACB=AEP\angle A F P=\angle A C B=\angle A E P, the points A,P,F,EA, P, F, E are concyclic. In addition, from EPA=EFA=DBA\angle E P A=\angle E F A=\angle D B A, points A,B,D,PA, B, D, P are concyclic.
By considering the radical centre of (BDFE),(APFE)(B D F E),(A P F E) and (BDPA)(B D P A), we find that the lines BD,AP,EFB D, A P, E F are concurrent at CC. The result follows.

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