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Geometry Difficulty 6.3 National olympiad Prove it Silk Road Mathematics Competition

Let MM be the midpoint of side ABAB in triangle ABCABC. B1B_1 is a point on segment ACAC such that CB=CB1CB = CB_1. The circumcircles of triangles ABCABC and BMB1BMB_1, ω\omega and ω1\omega_1, intersect for the second time at point KK. Let QQ be the midpoint of arc ACBACB of ω\omega. Lines B1QB_1Q and BCBC intersect at point EE. Prove that line KCKC passes through the midpoint of segment B1EB_1E.

Solution

Let the bisector of ACB\angle ACB intersect ω\omega for the second time at point NN. Note that NN is the midpoint of arc ABAB (that does not contain CC) of ω\omega. Also, it is easy to see that line CNCN is the perpendicular bisector of segment BB1BB_1. Thus, NA=NB=NB1NA = NB = NB_1, i.e. points A,BA, B and B1B_1 lie on a circle centered at NN with radius NANA. Since NAQ=NBQ=90\angle NAQ = \angle NBQ = 90^\circ, then lines QAQA and QBQB are tangent to this circle. Therefore, line QB1QB_1 contains the symmedian of triangle ABB1ABB_1 corresponding to vertex B1B_1.
Figure 1
Let line KCKC intersect ω1\omega_1 for the second time at point PP. From the properties of symmedian it follows that
(B1C,B1E)=(B1C,B1Q)=(B1M,B1B)=(PM,PB)=α.(1) \angle(B_1C, B_1E) = \angle(B_1C, B_1Q) = \angle(B_1M, B_1B) = \angle(PM, PB) = \alpha. \qquad (1)
Also, the following equalities hold:
(CK,CA)=(BK,BA)=(BK,BM)=(PK,PM)=β.(2) \angle(CK, CA) = \angle(BK, BA) = \angle(BK, BM) = \angle(PK, PM) = \beta. \qquad (2)
From (1) and (2) it follows that (PK,B1E)=(CK,B1E)=α+β=(PK,PB)\angle(PK, B_1E) = \angle(CK, B_1E) = \alpha + \beta = \angle(PK, PB). The latter equality gives
B1EPB.(3) B_1E \parallel PB. \qquad (3)
Let line BPBP intersect line ACAC at point RR. Then, from (2) it follows that CAPMCA \parallel PM, or that MPMP is a midsegment of triangle ARBARB, i.e. PP is the midpoint of segment BRBR. Now it is enough to note that if line KCKC bisects segment BRBR, then it also bisects segment B1EB_1E, since (3) holds.

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