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Geometry Difficulty 5.8 AIME, harder Prove it Iran

MM is an arbitrary point on the side BCBC of the triangle ABCABC. ω\omega is a circle tangent to the segments ABAB and BMBM in TT and KK respectively, also is tangent to the circumcircle of AMCAMC in PP. If TKAMTK \parallel AM, prove that the circumcircles of APTAPT and KPCKPC are tangent to each other.

Solution

It is obvious that the bisector of B\angle B is perpendicular to TKTK and so to the AMAM (by assumption). Thus BM=BABM = BA, and the circumcenter of AMCAMC is on the bisector of B\angle B (the perpendicular bisector of AMAM). Since the center of ω\omega is on the bisector of B\angle B, and PP is on the line connecting the centers of these two circles, it is on the bisector of B\angle B, too. Hence BAP=BMP=180PMC=PAC\angle BAP = \angle BMP = 180^\circ - \angle PMC = \angle PAC. So PP is on the bisector of A\angle A and hence PP is the incenter of ABCABC. So we have:
TPK=BTK=90B2=A2+C2=TAP+KCP \angle TPK = \angle BTK = 90^\circ - \frac{\angle B}{2} = \frac{\angle A}{2} + \frac{\angle C}{2} = \angle TAP + \angle KCP
So if we draw the ray PxPx such that TAP=TPx\angle TAP = \angle TPx and KCP=KPx\angle KCP = \angle KPx, then this ray is tangent to the circumcircles of ATPATP and KPCKPC, and according to the above equation such ray exists. Thus the circles APTAPT and KPCKPC are tangent in PP.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.