Maths Olympiad Prep

Library / /12 of 24

Geometry Difficulty 6.7 National olympiad Prove it Argentina

Let ABCABC be an acute-angled and scalene triangle, ω\omega its incircle and ω\omega' the excircle relative to the vertex AA. The circles ω\omega and ω\omega' are tangent to BCBC at PP and PP' respectively. Let Γ\Gamma be the circumference passing through BB and CC that is tangent to ω\omega in a point QQ, and Γ\Gamma' be the circumference passing through BB and CC that is tangent to ω\omega' in a point QQ'. The lines PQPQ and PQP'Q' intersect in NN. Prove that ANAN is perpendicular to BCBC.

Solution

Let SS be the intersection of the lines QRQR and BCBC.
Figure 1

First, note that the homothety with center QQ which transforms ω\omega into Γ\Gamma maps PP to a point in Γ\Gamma whose tangent is parallel to BCBC, namely, MM. Then, QQ, PP, MM are collinear. Then, the equality BQM=MQCBQM = MQC implies that QPQP is the internal bisector of BQCBQC and, since PQR=90PQR = 90^\circ, it follows that QSQS is the external bisector of BQCBQC.

Setting BC=aBC = a, CA=bCA = b, AB=cAB = c, and 2p=a+b+c2p = a+b+c, we have that BP=PC=pbBP = P'C = p-b, PC=pcPC = p-c and PP=bcPP' = b-c (in case b>cb > c; the other case is similar), and, by the angle bisector theorem, we have:
BSCS=BQCQ=BPCP=pbpc \frac{BS}{CS} = \frac{BQ}{CQ} = \frac{BP}{CP} = \frac{p-b}{p-c}
Then,
BSBC=BSCSBS=pbbc \frac{BS}{BC} = \frac{BS}{CS - BS} = \frac{p-b}{b-c}
or, equivalently, BS=a(pb)bcBS = \frac{a(p-b)}{b-c}.

Let SS be the area, rr the inradius, and rAr_A the AA-exradius of the triangle ABCABC.
We have that RPS=PQS=PJ=90RPS = PQS = P'J = 90^\circ; hence, PQS=RPS=PJPPQS = RPS = P'JP. Therefore, PSRPJPPSR \sim P'JP, which implies that:
SPRP=PJPPor, equivalently,a(pb)bc+(pb)2r=PJbc \frac{SP}{RP} = \frac{P'J}{PP'} \quad \text{or, equivalently,}\quad \frac{\frac{a(p-b)}{b-c} + (p-b)}{2r} = \frac{P'J}{b-c}
and, as a consequence,
PJ=(a+bc)(pb)2r=(pc)(pb)S/p=p(pb)(pc)S=Spa=rA P'J = \frac{(a+b-c)(p-b)}{2r} = \frac{(p-c)(p-b)}{S/p} = \frac{p(p-b)(p-c)}{S} = \frac{S}{p-a} = r_A
(here, we use that S=p(pa)(pb)(pc)S = \sqrt{p(p-a)(p-b)(p-c)}).

Therefore, JJ is the center of the excircle of ABCABC relative to the vertex AA.

Similarly, it can be proved that PQP'Q' passes through the incenter II of ABCABC. (To do this, it suffices to show that QPQ'P' is the internal bisector of BQCBQ'C and that, if RR' is the point diametrically opposite to PP' in ω\omega', then RQR'Q' is the external bisector of BQCBQ'C. Then, if SS' is the intersection point of RQR'Q' and BCBC we can show as before that PIPPSRP'IP \sim P'S'R').
Figure 2

Now we will prove that both lines JPJP and IPIP' pass through the midpoint TT of the altitude AHAH of the triangle ABCABC. This finishes the problem, since T=NT = N would be the intersection of the lines JP=PQJP = PQ and IP=PQIP' = P'Q', and ATAT is perpendicular to BCBC. The homothety with center AA that transforms ω\omega into ω\omega', transforms the diameter RPRP into the diameter PRP'R'. Since AHPRPPAHP \sim R'P'P (since AHPRAH \parallel P'R'), and PTPT, PJPJ are medians relative to the corresponding parallel sides AHAH and PRP'R', then TT, PP and JJ are collinear. In addition, since AHPRPPAHP' \sim RPP' (because AHRPAH \parallel RP) and PIP'I, PTP'T are medians relative to the corresponding parallel sides AHAH and RPRP, then TT, PP', II are also collinear. Therefore, JPJP and IPIP' both pass through TT, as we wanted to prove.

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 and solution reproduced as published; topic and difficulty added by this site.