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Geometry Difficulty 6.3 National Olympiad Prove it Czech Republic

Let ABCABC be an acute scalene triangle. Let DD and EE be points on the sides ABAB and ACAC, respectively, such that BD=CEBD = CE. Denote by O1O_1 and O2O_2 the circumcentres of the triangles ABEABE and ACDACD, respectively. Prove that the circumcircles of the triangles ABCABC, ADEADE and AO1O2AO_1O_2 have a common point different from AA.

Solutions — 3

Solution 1

Let ZZ be the midpoint of the longer arc BCBC of the circumcircle ω\omega of the triangle ABCABC. The triangles ZDBZDB and ZECZEC are congruent, because they agree in the sides BD=CEBD = CE and ZB=ZCZB = ZC, as well as in the corresponding angles between them, for both lie over the chord AZAZ of ω\omega. It follows that ZDA=ZEA\angle ZDA = \angle ZEA, which in turn discloses that the quadrilateral ADEZADEZ is cyclic. So it remains to be shown that the quadrilateral AO1O2ZAO_1O_2Z is cyclic.

Figure 1

The center OO of ω\omega satisfies OO1ABOO_1 \perp AB and OO2ACOO_2 \perp AC. The projections of OO and O1O_1 onto ACAC are the midpoints of ACAC and AEAE respectively. Thus the projection of the segment OO1OO_1 onto ACAC has length 12CE\frac{1}{2}CE. For the same reason, the projection of OO2OO_2 on ABAB has length 12BD\frac{1}{2}BD, and by hypothesis these two length agree. Moreover, the angle between OO1OO_1 and ACAC is the same as the angle between OO2OO_2 and ABAB. It follows that OO1=OO2OO_1 = OO_2.
Further, we have AOO1=ACB=O2OZ\angle AOO_1 = \angle ACB = \angle O_2OZ, the latter being a consequence of ZOBCZO \perp BC and OO2ACOO_2 \perp AC. So the rays OAOA and OZOZ are isogonal in the angle O2OO1O_2OO_1. In the combination with AO=ZOAO = ZO and OO1=OO2OO_1 = OO_2 this proves that

the quadrilateral AO1O2ZAO_1O_2Z is an isosceles trapezium and thus in particular cyclic.
Thereby the problem is solved.

Solution 2

Let the circumcircles of triangles ABEABE and ADCADC intersect each other again at FAF \neq A. Then the triangles BFDBFD and EFCEFC are congruent, for they agree in their sides BD=CEBD = CE as well as in their corresponding adjacent angles, i.e., FBD=FEC\angle FBD = \angle FEC and BDF=ECF\angle BDF = \angle ECF. It follows that the altitudes of these triangles passing through FF have the same lengths, wherefore AFAF is the bisector of the angle BACBAC.

Figure 2

Now construct the point SS such that SO1FO2SO_1FO_2 is a parallelogram. We will show that SS is the desired point.
To prove that SS lies on the circumcircle of triangle AO1O2AO_1O_2, we note that the triangles AO1O2AO_1O_2 and FO1O2FO_1O_2 are congruent due to AO1=FO1AO_1 = FO_1 and AO2=FO2AO_2 = FO_2. It follows that AO1O2SAO_1O_2S is an isosceles trapezium and hence in particular a cyclic quadrilateral, as claimed. Later, it will help us to have observed that the facts used in this paragraph imply ASO1O2AFAS \parallel O_1O_2 \perp AF.
Next, we prove that SS lies on the circumcircle of the triangle ABCABC and that it is actually the midpoint of its longer arc BCBC; this will also show SAS \neq A, as needed. Our first intermediate step is to observe that the triangles O1SBO_1SB and O2CSO_2CS are congruent. Indeed they agree in a pair of sides, BO1=FO1=SO2BO_1 = FO_1 = SO_2 and SO1=FO2=CO2SO_1 = FO_2 = CO_2. Moreover the corresponding angles between these sides are equal, because their complements to 360360^\circ are equal as a consequence of
BO1F=2BAF=2FAC=FO2C \angle BO_1F = 2\angle BAF = 2\angle FAC = \angle FO_2C
and FO1S=SO2F\angle FO_1S = \angle SO_2F. This concludes the verification of O1SBO2CS\triangle O_1SB \cong \triangle O_2CS, and it follows that BS=CSBS = CS. Further, since AFAF is the bisector of BAC\angle BAC and AFASAF \perp AS, the line ASAS is the exterior bisector of BAC\angle BAC. Altogether we obtain that SS is the point described above. The fact that SS lies on the circumcircle of the triangle ADEADE can be shown as in the first solution.

Solution 3

Denote by OO and PP the circumcentres of triangles ABCABC and ADEADE, respectively. The lines OO1OO_1 and O2PO_2P (being the perpendicular bisectors of ABAB and ADAD, respectively) are both perpendicular to ABAB and their distance is 12BD\frac{1}{2}BD. Similarly, the lines OO2OO_2 and O1PO_1P are both perpendicular to ACAC and their distance is 12CE\frac{1}{2}CE. Since 12BD=12CE\frac{1}{2}BD = \frac{1}{2}CE, the quadrilateral O1OO2PO_1OO_2P is a parallelogram with equal altitudes, hence a rhombus. It follows that OPOP is the perpendicular bisector of O1O2O_1O_2, so all the three circumcentres of the triangles ABCABC, ADEADE and AO1O2AO_1O_2 lie on the same line, which concludes the claim (since AA does not lie on this line because of ABACAB \neq AC).

Figure 3

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