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Romanian Master of Mathematics

Geometry Difficulty 7.7 National Olympiad, round 2 Prove it Romania

Let BCBC be a fixed segment in the plane, and let AA be a variable point in the plane not on the line BCBC. Distinct points XX and YY are chosen on the rays CA⃗\vec{CA} and BA⃗\vec{BA}, respectively, such that ∠CBX=∠YCB=∠BAC\angle CBX = \angle YCB = \angle BAC. Assume that the tangents to the circumcircle of ABCABC at BB and CC meet line XYXY at PP and QQ, respectively, such that the points X,P,YX, P, Y, and QQ are pairwise distinct and lie on the same side of BCBC. Let Ω1\Omega_1 be the circle through XX and PP centred on BCBC. Similarly, let Ω2\Omega_2 be the circle through YY and QQ centred on BCBC. Prove that Ω1\Omega_1 and Ω2\Omega_2 intersect at two fixed points as AA varies.
Denmark, Daniel Pham Nguyen

Solution

Let X′,Y′,P′X', Y', P', and Q′Q' be the reflections across BCBC of X,Y,PX, Y, P, and QQ, respectively. Then Ω1\Omega_1 and Ω2\Omega_2 are just the circles PXX′P′PXX'P' and QYY′Q′QYY'Q', respectively.
Denote α=∠BAC=∠CBX=∠YCB\alpha = \angle BAC = \angle CBX = \angle YCB. Let XYXY cross BCBC at WW; the case XY∥BCXY \parallel BC may be treated as a limit case. The symmetry yields that WW also lies on the line X′Y′X'Y'. The same symmetry, along with tangency of PBPB and QCQC to the circle ABCABC, yields
α=∠X′BC=∠PBW=∠WCQ=∠BCY′.(∗) \alpha = \angle X'BC = \angle PBW = \angle WCQ = \angle BCY'. \quad (*)
This yields that each of the triples (P,B,X′)(P, B, X'), (Q,C,Y′)(Q, C, Y'), (P′,B,X)(P', B, X), and (Q′,C,Y)(Q', C, Y) is collinear, and, moreover, that PBX′∥Q′CYPBX' \parallel Q'CY and P′BX∥QCY′P'BX \parallel QCY'. It follows now that quadrilaterals PXX′P′PXX'P' and YQQ′Y′YQQ'Y' are homothetic at WW. Therefore, so are Ω1\Omega_1 and Ω2\Omega_2.

Let now Ω1\Omega_1 and Ω2\Omega_2 cross at DD and D′D'. Let WDWD and WD′WD' meet Ω1\Omega_1 again at EE and E′E'. Since W=PX∩P′X′W = PX \cap P'X' and B=PX′∩P′XB = PX' \cap P'X, the point BB lies on the polar of WW with respect to Ω1\Omega_1. In other words, WW and BB are inverse with respect to that circle. This yields that the lines DE′DE' and D′ED'E also cross at BB.
Figure 1

Now, we have ∠BDX=∠E′DX=∠X′D′E=∠X′PE=∠BPE=∠CYD\angle BDX = \angle E'DX = \angle X'D'E = \angle X'PE = \angle BPE = \angle CYD (the last equality holds by means of homothety). Similarly, we have ∠DXB=∠DXP′=∠PX′D′=∠PED′=∠PEB=∠YDC\angle DXB = \angle DXP' = \angle PX'D' = \angle PED' = \angle PEB = \angle YDC. Therefore, the triangles BDXBDX and CYDCYD are similar. Firstly, this yields that ∠DBC=∠DBX+∠XBC=∠YCD+∠BCY=∠BCD\angle DBC = \angle DBX + \angle XBC = \angle YCD + \angle BCY = \angle BCD, whence BD=CDBD = CD. Secondly, this also implies that BD/BX=CY/CDBD/BX = CY/CD, or BX⋅CY=BD⋅CD=BD2BX \cdot CY = BD \cdot CD = BD^2. But the triangles BXCBXC and CBYCBY are also similar (as both are similar to ABCABC), so BX/BC=BC/CYBX/BC = BC/CY, or BX⋅CY=BC2BX \cdot CY = BC^2. Thus, BC=BD=CDBC = BD = CD, and the triangle BCDBCD is equilateral. This finishes the solution.

Solution 2:
All angles in the solution are directed. All segment lengths on lines BXBX and CYCY (and parallel to them) are also oriented; we assume that the directions BX→\overrightarrow{BX} and CY→\overrightarrow{CY} are positive. As in the solution above, we prove that BP∥CYBP \parallel CY.
Assume that ABCABC is oriented anti-clockwise. Let DD and D′D' be the points such that the triangles DBCDBC and D′CBD'CB are equilateral, and oriented anti-clockwise. We will show that DD and D′D' lie on the circle Ω1\Omega_1; similarly, they lie on Ω2\Omega_2.
Notice that α=∠BAC=∠CBX=∠YCB=π−∠CBP\alpha = \angle BAC = \angle CBX = \angle YCB = \pi - \angle CBP; moreover, each of the triangles XBCXBC and BCYBCY is similar to BACBAC and oriented differently than BACBAC; hence those two triangles are equi-oriented. Let Ω\Omega denote the circle (DD′X)(DD'X); clearly, its centre lies on the perpendicular bisector of DD′DD', i.e., on BCBC. We aim to prove that Ω\Omega passes through PP; that will yield that Ω=Ω1\Omega = \Omega_1, which establishes what we are aimed to prove.
Denote Z=XB∩YCZ = XB \cap YC. Since ∠CBZ=∠BAC=∠ZCB\angle CBZ = \angle BAC = \angle ZCB, we have ZB=ZCZB = ZC, and hence ZZ lies on the perpendicular bisector DD′DD' of BCBC. By similarity, we get BX/BC=BC/CYBX/BC = BC/CY, or BC2=BX⋅CY=BX⋅(ZY+CZ)BC^2 = BX \cdot CY = BX \cdot (ZY + CZ). Since CY∥BPCY \parallel BP, the triangles XZYXZY and XBPXBP are similar, so BX⋅ZY=ZX⋅BPBX \cdot ZY = ZX \cdot BP.
Figure 2
Therefore, BD2=BC2=BX⋅ZY+BX⋅CZ=ZX⋅BP+(BZ+ZX)⋅BZ=ZX⋅(BP+BZ)+BZ2BD^2 = BC^2 = BX \cdot ZY + BX \cdot CZ = ZX \cdot BP + (BZ + ZX) \cdot BZ = ZX \cdot (BP + BZ) + BZ^2.
On the other hand, let MM be the midpoint of BCBC, and let XBXB cross Ω\Omega again at P′P'. Write the power of point ZZ with respect to Ω\Omega as XZ⋅(P′B+BZ)=XZ⋅P′Z=ZD⋅ZD′=MZ2−DM2=BZ2−MB2−DM2=BZ2−BD2XZ \cdot (P'B + BZ) = XZ \cdot P'Z = ZD \cdot ZD' = MZ^2 - DM^2 = BZ^2 - MB^2 - DM^2 = BZ^2 - BD^2.
The two obtained relations yield
ZX⋅(BP+BZ)=BD2−BZ2=ZX⋅(P′B+BZ), ZX \cdot (BP + BZ) = BD^2 - BZ^2 = ZX \cdot (P'B + BZ),
so BP=P′BBP = P'B, and so PP and P′P' are reflections of one another in the line BCBC. Thus, PP lies on Ω\Omega, as desired.

Remarks.
(1) It is also possible to solve the problem via the moving points method. Introduce the points DD and D′D' as in Solution 2, and introduce the reflections X′X', Y′Y', P′P', and Q′Q' of XX, YY, PP, and QQ in the line BCBC, respectively, as in Solution 1, to read Ω1\Omega_1 and Ω2\Omega_2 as the circles PXX′P′PXX'P' and QYY′Q′QYY'Q', respectively.
We need to show that DD lies on Ω2\Omega_2 (the other incidences are similar). To this end, it suffices to check that ∠YDQ=∠YY′Q=90∘−∠Y′CB=90∘−∠BAC\angle YDQ = \angle YY'Q = 90^\circ - \angle Y'CB = 90^\circ - \angle BAC.
Fix BB, CC, and the circle ABCABC. As AA varies on that circle, the lines BXBX, CYCY, BPBP, and CQCQ remain constant, and XX and YY depend projectively on AA. Choosing Q1Q_1 on CQCQ such that ∠YDQ1=90∘−∠BAC\angle YDQ_1 = 90^\circ - \angle BAC, we need to show that Q1=QQ_1 = Q, or that XX, YY, and Q1Q_1 are collinear. The point Q1Q_1 also depends projectively on AA, so it suffices to check that the points Q1Q_1, XX, and YY are collinear for four specific positions of AA.

(2) Although inversion, homothety, and the moving point method are essentially the same thing, i.e., a Möbius transformation, we briefly sketch yet another approach below:
Let BP∩CQ=DBP \cap CQ = D, BX∩CY=ZBX \cap CY = Z, and DZ∩XY=TDZ \cap XY = T. Let R1R_1 and R2R_2 be distinct points such that triangles R1BCR_1BC and R2BCR_2BC are equilateral. We first note that BZ∥CDBZ \parallel CD and CZ∥BDCZ \parallel BD, so the triangles DPQDPQ and ZYXZYX are homothetic from TT. Hence, TP⋅TX=TQ⋅TYTP \cdot TX = TQ \cdot TY, so TT lies on the radical axis of Ω1\Omega_1 and Ω2\Omega_2. Since their centres both lie on BCBC, their radical axis is the perpendicular from TT to BCBC, i.e. the perpendicular bisector DZDZ of BCBC. As CY∥DPCY \parallel DP and BX∥DQBX \parallel DQ, it follows that BP=YZ⋅BXXZBP = YZ \cdot \frac{BX}{XZ} and CQ=XZ⋅CYYZCQ = XZ \cdot \frac{CY}{YZ}. As triangles BXCBXC and CYBCYB are similar, this means that BP⋅CQ=BX⋅CY=BC2BP \cdot CQ = BX \cdot CY = BC^2.
Perform an inversion about Ω\Omega, the circle of radius BCBC centred at BB. Let X′X', P′P', and Ω1′\Omega'_1 denote the images of XX, PP and Ω1\Omega_1. As BX⋅CY=BP⋅CQ=BC2BX \cdot CY = BP \cdot CQ = BC^2, we get BP′=CQBP' = CQ and BX′=CYBX' = CY. Since lines BDBD, CDCD and BZBZ, CZCZ are symmetric about DZDZ, it follows that X′X', P′P' and YY, QQ are symmetric about DZDZ. Thus, Ω1′\Omega'_1 and Ω2\Omega_2 are reflections about DZDZ. Since Ω1∩Ω2\Omega_1 \cap \Omega_2 and Ω1′∩Ω2\Omega'_1 \cap \Omega_2 all lie on DZDZ, it follows that Ω\Omega, Ω1\Omega_1, Ω1′\Omega'_1, Ω2\Omega_2 all meet on DZDZ. This implies that Ω1\Omega_1 and Ω2\Omega_2 both pass through R1R_1 and R2R_2, the two required fixed points.

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