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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 P′P' 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 Q′Q'. The lines PQPQ and P′Q′P'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=90∘PQR = 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=P′C=p−bBP = P'C = p-b, PC=p−cPC = p-c and PP′=b−cPP' = b-c (in case b>cb > c; the other case is similar), and, by the angle bisector theorem, we have:
BSCS=BQCQ=BPCP=p−bp−c \frac{BS}{CS} = \frac{BQ}{CQ} = \frac{BP}{CP} = \frac{p-b}{p-c}
Then,
BSBC=BSCS−BS=p−bb−c \frac{BS}{BC} = \frac{BS}{CS - BS} = \frac{p-b}{b-c}
or, equivalently, BS=a(p−b)b−cBS = \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=P′J=90∘RPS = PQS = P'J = 90^\circ; hence, PQS=RPS=P′JPPQS = RPS = P'JP. Therefore, PSR∼P′JPPSR \sim P'JP, which implies that:
SPRP=P′JPP′or, equivalently,a(p−b)b−c+(p−b)2r=P′Jb−c \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,
P′J=(a+b−c)(p−b)2r=(p−c)(p−b)S/p=p(p−b)(p−c)S=Sp−a=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(p−a)(p−b)(p−c)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 P′Q′P'Q' passes through the incenter II of ABCABC. (To do this, it suffices to show that Q′P′Q'P' is the internal bisector of BQ′CBQ'C and that, if R′R' is the point diametrically opposite to P′P' in ω′\omega', then R′Q′R'Q' is the external bisector of BQ′CBQ'C. Then, if S′S' is the intersection point of R′Q′R'Q' and BCBC we can show as before that P′IP∼P′S′R′P'IP \sim P'S'R').
Figure 2

Now we will prove that both lines JPJP and IP′IP' 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′=P′Q′IP' = 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 P′R′P'R'. Since AHP∼R′P′PAHP \sim R'P'P (since AH∥P′R′AH \parallel P'R'), and PTPT, PJPJ are medians relative to the corresponding parallel sides AHAH and P′R′P'R', then TT, PP and JJ are collinear. In addition, since AHP′∼RPP′AHP' \sim RPP' (because AH∥RPAH \parallel RP) and P′IP'I, P′TP'T are medians relative to the corresponding parallel sides AHAH and RPRP, then TT, P′P', II are also collinear. Therefore, JPJP and IP′IP' both pass through TT, as we wanted to prove.

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