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Geometry Difficulty 6.0 National olympiad Prove it Belarus

The altitudes CC1CC_1 and BB1BB_1 are drawn in the acute triangle ABCABC. The bisectors of angles BB1C\angle BB_1C and CC1B\angle CC_1B intersect the line BCBC at points DD and EE respectively and meet each other at point XX.
Prove that the intersection points of circumcircles of the triangles BEXBEX and CDXCDX lie on the line AXAX.

Solutions — 2

Solution 1

Since BB1C=CC1B=90\angle BB_1C = \angle CC_1B = 90^\circ, the points B,C1,B1B, C_1, B_1 and CC lie on the circle ω\omega with the diameter BCBC. Hence the bisectors of angles BB1C\angle BB_1C and CC1B\angle CC_1B pass through the midpoint of the arc BCBC of ω\omega, so this midpoint is XX.

Figure 1

First we prove that the quadrilateral EC1B1DEC_1B_1D is cyclic. The angle BDB1\angle BDB_1 is an external angle of the triangle CB1DCB_1D, hence BDB1=ACB+45\angle BDB_1 = \angle ACB + 45^\circ. Since BB1C\angle BB_1C is right, B1BC=90ACB\angle B_1BC = 90^\circ - \angle ACB. Thus,
B1C1E=B1C1C+45=135ACB. \angle B_1C_1E = \angle B_1C_1C + 45^\circ = 135^\circ - \angle ACB.
B1C1E+EDB1=180. \angle B_1C_1E + \angle EDB_1 = 180^\circ.
Now we prove that the intersection points of the circumcircles of the triangles AB1DAB_1D and AC1EAC_1E lie on the line AXAX. Let the circumcircle of the triangle AB1DAB_1D intersect the line AXAX at points AA and PP, then XPXA=XDXB1XP \cdot XA = XD \cdot XB_1. Since points E,C1,B1E, C_1, B_1 and DD are concyclic, XEXC1=XDAB1XE \cdot XC_1 = XD \cdot AB_1. Hence XEXC1=XPXAXE \cdot XC_1 = XP \cdot XA, and therefore the circumcircle of the triangle AC1EAC_1E passes through PP.

Finally, we will prove that the circumcircles of the triangles BEXBEX and CDXCDX passes through points XX and PP. Since point XX is the midpoint of the arc BCBC, XB=XCXB = XC and BXC=90\angle BXC = 90^\circ. Therefore, BCX=CBX=45\angle BCX = \angle CBX = 45^\circ. Since the quadrilateral AB1DPAB_1DP is cyclic, APD=180DB1A=45\angle APD = 180^\circ - \angle DB_1A = 45^\circ. Hence DPX+DCX=180\angle DPX + \angle DCX = 180^\circ, i.e. points P,D,CP, D, C and XX are concyclic. Similarly, the quadrilateral BEPXBEPX is cyclic. If point PP is distinct from point XX, the problem is solved.
If points PP and XX coincide, the equalities DCX=APD\angle DCX = \angle APD and EBX=APE\angle EBX = \angle APE imply that the circumcircles of the triangles BEXBEX and CDXCDX are tangent to the line AXAX.

Solution 2

Note that BC1B1CBC_1B_1C is concyclic, and let ω,ωB\omega, \omega_B and ωC\omega_C be the circumcircles of BC1B1CBC_1B_1C, BEXBEX, and CDXCDX, respectively. Since B1XB_1X and C1XC_1X are the bisectors of BB1C\angle BB_1C and CC1B\angle CC_1B, they both pass through the midpoint of the arc BCBC of ω\omega not containing B1B_1 and C1C_1, thus this midpoint is XX. Let AXAX intersect ω,ωB\omega, \omega_B and ωC\omega_C the second time at points T,FBT, F_B and FCF_C, respectively, and denote AXBC=PAX \cap BC = P.

As in the first solution, note that EC1B1DEC_1B_1D is cyclic. Let γ,γB\gamma, \gamma_B, and γC\gamma_C be the circumcircles of EC1B1DEC_1B_1D, BEC1BEC_1, and CDB1CDB_1, respectively. Since C1EC_1E and B1DB_1D are radical axes of γB,γ\gamma_B, \gamma and γB,γ\gamma_B, \gamma and C1EB1D=XC_1E \cap B_1D = X, the radical axis of γB,γC\gamma_B, \gamma_C passes through XX. On the other hand, it passes through AA since PowγBA=AC1AB=AB1AC=PowγCA\text{Pow}_{\gamma_B} A = AC_1 \cdot AB = AB_1 \cdot AC = \text{Pow}_{\gamma_C} A. Thus, AXAX is the radical axis of γB,γC\gamma_B, \gamma_C, and it follows from PAXP \in AX that
PFAPX=PDPC=PEPB=PFBPX. PF_A \cdot PX = PD \cdot PC = PE \cdot PB = PF_B \cdot PX.
Consequently, FA=FBF_A = F_B and the problem is solved.

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