Maths Olympiad Prep

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Geometry Difficulty 6.3 National olympiad Prove it Belarus

Let ABCABC be a triangle such that CAB=30\angle CAB = 30^\circ and ACB=60\angle ACB = 60^\circ. An arbitrary point DD is chosen on the extension of the ray ABAB beyond point BB. The point EE lies on the extension of the ray CBCB beyond the point BB such that BDE=60\angle BDE = 60^\circ. The lines ACAC and DEDE intersect at FF.
Prove that the circumcircle of the triangle AEFAEF passes through some fixed point different from AA and not depending on the choice of DD.
(Mikhail Karpuk)

Solution

Let GG be the point on the ray BCBC such that BG=3BCBG = 3BC. We will show that GG lies on the circumcircle of the triangle AEFAEF. Angles ECF\angle ECF and ACG\angle ACG are equal 120120^\circ as they are adjacent to ACB=60\angle ACB = 60^\circ. Since BED=30\angle BED = 30^\circ and FCE=120\angle FCE = 120^\circ, CFE=30\angle CFE = 30^\circ. Hence the triangle FCEFCE is isosceles with equal sides CF=CECF = CE. The leg CBCB opposite to the angle 3030^\circ is equal to half of the hypotenuse ACAC, so AC=2CB=BGCB=CGAC = 2CB = BG - CB = CG. Hence the triangle GCAGCA is isosceles, CG=CACG = CA and GAC=AGC=30\angle GAC = \angle AGC = 30^\circ. Note that the common bisector \ell of the angles ACGACG and ECFECF is also the common perpendicular bisector of the segments AGAG and EFEF. Hence the triangles AEFAEF and GFEGFE are symmetrical to each other with respect to \ell, and the centers of their circumcircles lie on line of symmetry. So they have a common circumscribed circle, i. e. point GG lies on the circumscribed circle of triangle AEFAEF.

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