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Geometry Difficulty 6.5 National Olympiad Prove it Iran

In triangle ABC the points N and M are on the side BCBC such that BAM=MAN=NAC\angle BAM = \angle MAN = \angle NAC. The points P and Q are on the angle bisector such that A, P, and Q are all on the same side with respect to BCBC and 13BAC=12BPC=BQC\frac{1}{3}\angle BAC = \frac{1}{2}\angle BPC = \angle BQC. Let AMAM and CQCQ meet at EE and ANAN and BQBQ meet at FF, prove that the common tangents of the circumcircles of FPEFPE and FQEFQE intersect on the circumcircle of ABCABC.

Solution

We shall prove the following lemma.

Figure 1

Lemma 1. *The points EE, FF, BB, and CC are on a circle centered at TT'.*

Proof. An easy angle chasing shows that BAEQBAEQ and CAFQCAFQ are indeed cyclic.
Thus,
FBE=QBE=QAE=QAF=QCF=FCE. \angle FBE = \angle QBE = \angle QAE = \angle QAF = \angle QCF = \angle FCE.
Yielding EFBCEFBC is cyclic. Then,
BEC=BQE+EBQ=α+FBE=α+QBF=3α2 \angle BEC = \angle BQE + \angle EBQ = \alpha + \angle FBE = \alpha + \angle QBF = \frac{3\alpha}{2}
and
BTC=BAC=3α=2BEC \angle BT'C = \angle BAC = 3\alpha = 2\angle BEC
And
BT=CT. BT' = CT'.

Now, we need the following lemma;

Lemma 2. *The quadrilateral EFATEFA'T is cyclic*

Proof. According to the above-mentioned lemma; FTE=2FBE=2QBF=2QAE=EAF\angle FT'E = 2\angle FBE = 2\angle QBF = 2\angle QAE = \angle EAF. This completes our proof.

Figure 2

Thus the radical axis of ATEFAT'EF, ATCBAT'CB, and EFBCEFBC are concurrent. Let GG be the foot of ex-angle bisector of BAC\angle BAC, and assume BEBE and CFCF meet at PP'. Since BCBC and GPGP' meet at XX, it follows that PP' is on ADAD, indeed (GX,BC)=(GD,BC)=1(GX, BC) = (GD, BC) = -1. Yielding X=DX = D. That is;
BPC=BEC+FCE=BEC+FBE=2α \angle BP'C = \angle BEC + \angle FCE = \angle BEC + \angle FBE = 2\alpha
Yielding to the fact that PP is the intersection of the corresponding arcs of ADAD and BCBC. Therefore, P=PP = P'. Whence, CFBE=DCF \cap BE = D. This implies that EPF=BPC=2α\angle EPF = \angle BPC = 2\alpha. According to the first lemma, ETF=EAF\angle ET'F = \angle EAF and TT' is on the perpendicular bisector of EFEF. Let RR be the intersection point of the common tangents of QEFQEF and PEFPEF, taking into account that RR is the center of the homothety of these circles, it follows that ERF=EPFEQF=α\angle ERF = \angle EPF - \angle EQF = \alpha. Therefore, R,O1R, O_1, and O2O_2 would be collinear and RR is on the perpendicular bisector of O1O2O_1O_2. Hence, RR is the intersection of the corresponding arc of the EFEF and its perpendicular bisector. Since such a point would be unique, we find that R=TR = T'. We are done. ■

Figure 2

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