Maths Olympiad Prep

Library / /56 of 62

Geometry Difficulty 7.1 National Olympiad, round 2 Prove it Ukraine

Let AMAM be a median in an acute triangle ABCABC. Its extension intersect the circumcircle ww of ABCABC at PP. Let AH1AH_1 be an altitude of ABC\triangle ABC, HH - its orthocenter. The rays MHMH and PH1PH_1 intersect ww at KK and TT respectively. Prove that the circumcircle of AKTH1\triangle AKTH_1 is tangent to BCBC.

(Khilko Danylo)

Solution

It suffices to show that TKH1=TH1B\angle TKH_1 = \angle TH_1B (Fig. 14). Let us extend KH1KH_1 and intersect if with ww at SS. Then
TKS=TAB+BAS,TH1B=TAB+PAC, \angle TKS = \angle TAB + \angle BAS, \quad \angle TH_1B = \angle TAB + \angle PAC,
So it is sufficient to show that PAC=BAS\angle PAC = \angle BAS.
Denote by A1A_1 the point such that AA1AA_1 is the diameter of ww. Then A1CA=ABA1=90\angle A_1CA = \angle ABA_1 = 90^\circ, so BHA1CBH \parallel A_1C and CHA1BCH \parallel A_1B. Then BHCA1BHCA_1 is a parallelogram, so HA1HA_1 passes through the point MM.

Then KK lies on HA1HA_1, hence A1KA=90\angle A_1KA = 90^\circ. Then the quadrilateral AKH1MAKH_1M is inscribed. So KH1A=KMA\angle KH_1A = \angle KMA. Suppose AH1AH_1 intersect ww secondly at FF. Then
KH1A=KCA+FAS,KMA=KCA+PAA1. \angle KH_1A = \angle KCA + \angle FAS, \quad \angle KMA = \angle KCA + \angle PAA_1.
Hence, FAS=PAA1\angle FAS = \angle PAA_1, also ABC=AA1C\angle ABC = \angle AA_1C, so we derive BAF=90ABC=A1AC\angle BAF = 90^\circ - \angle ABC = \angle A_1AC. Then
BAS=BAF+FAS=PAA1+A1AC=PAC. \angle BAS = \angle BAF + \angle FAS = \angle PAA_1 + \angle A_1AC = \angle PAC.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

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