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Geometry Difficulty 6.0 National olympiad Prove it Slovenia

The diagonal ACAC of a convex quadrilateral ABCDABCD is the bisector of the angle DCB\angle DCB. Let EE be the intersection of the side ABAB and the circumcircle of the triangle ACDACD. Let FF be the intersection of the side ADAD and the circumcircle of the triangle ABCABC. Prove that the segments AC,DEAC, DE and BFBF intersect in a point.

Solution

Let GG be the intersection of the segments ACAC and BFBF. The points A,B,CA, B, C and FF lie on the same circle, so AFB=ACB\angle AFB = \angle ACB. The diagonal ACAC bisects the angle DCB\angle DCB, so ACB=DCA\angle ACB = \angle DCA. This implies AFG=DCA\angle AFG = \angle DCA and GFD+DCG=π\angle GFD + \angle DCG = \pi. We conclude that the points C,D,FC, D, F and GG lie on the same circle.

Figure 1

The points A,E,CA, E, C and DD are concyclic, so AEC=πADC\angle AEC = \pi - \angle ADC and CEB=πAEC=ADC\angle CEB = \pi - \angle AEC = \angle ADC. Since CDFGCDFG is a cyclic quadrilateral, we have FGC=πFDC\angle FGC = \pi - \angle FDC and FGA=FDC\angle FGA = \angle FDC. So, BGC=FGA=FDC\angle BGC = \angle FGA = \angle FDC. We have shown that CEB=ADC\angle CEB = \angle ADC and BGC=ADC\angle BGC = \angle ADC, so CEB=BGC\angle CEB = \angle BGC. This implies that BCGEBCGE is a cyclic quadrilateral. So, BEG=πACB\angle BEG = \pi - \angle ACB, or GEA=ACB\angle GEA = \angle ACB. At the same time we have ACB=ACD\angle ACB = \angle ACD because ACAC is the bisector of the angle DCB\angle DCB. Since AECDAECD is a cyclic quadrilateral, we have ACD=AED\angle ACD = \angle AED. We conclude that GEA=DEA\angle GEA = \angle DEA and the points E,GE, G and DD are collinear. Hence, AC,BFAC, BF and DEDE meet at a point.

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