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Geometry Difficulty 6.1 National olympiad Prove it Greece

Let ABC\triangle ABC be a scalene triangle with AB<ACAB < AC, with circumcircle c(O,R)c(O, R). The circle c1(A,AB)c_1(A, AB) intersects the side BCBC at EE and the circle cc at FF. The line EFEF meets for a second time the circle cc at point DD and the side ACAC at point MM. The line ADAD intersects the side BCBC at point KK. Finally, the circumcircle of the triangle BKDBKD intersects ABAB at LL. Prove that the points K,L,MK, L, M lie on a line parallel to the line BFBF.

Solutions — 2

Solution 1

The angle F^1\hat{F}_1 is inscribed into the circle c1c_1 with corresponding central angle BAE^\hat{BAE}. Hence:
Figure 1
Figure 4
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F^1=BAE^2=A^1+A^22(1) \hat{F}_1 = \frac{BA\hat{E}}{2} = \frac{\hat{A}_1 + \hat{A}_2}{2} \qquad (1)
From the cyclic quadrilateral AFDB we have:
F^1=A^1(2) \hat{F}_1 = \hat{A}_1 \qquad (2)
From (1) and (2) we get that A^1=A^2\hat{A}_1 = \hat{A}_2, that is AKAK is the bisector of the angle BAE^BA\hat{E}. Since AB=AEAB = AE, we conclude that AKAK and hence DKDK is perpendicular to BCBC, that is
DKBC(3) DK \perp BC \qquad (3)
In the circle cc, cords ABAB and AFAF are equal, as radii of circle c1c_1 and so D^1=C^\hat{D}_1 = \hat{C}. Hence the quadrilateral DKMCDKMC is inscribable. Therefore we have DK^C=DM^C=90D\hat{K}C = D\hat{M}C = 90^\circ, that is:
DMAC(4) DM \perp AC \qquad (4)
From the inscribed quadrilateral BKDLBKDL we have BK^D=BL^D=90B\hat{K}D = B\hat{L}D = 90^\circ, that is:
DLAB(5) DL \perp AB \qquad (5)
From the relations (3), (4) and (5) we conclude that the points K,L,MK,L,M are on the Simson's of the triangle ABCABC corresponding to the point DD.
From the inscribable quadrilateral DKMCDKMC we get that M^1=C^1\hat{M}_1 = \hat{C}_1. Also from the inscribed quadrilateral ABDCABDC we have C^1=A^1\hat{C}_1 = \hat{A}_1 and finally from the inscribed quadrilateral ABDFABDF we have A^1=F^1\hat{A}_1 = \hat{F}_1. Hence M^1=F^1\hat{M}_1 = \hat{F}_1 from which we get that BF//LMBF // LM.

Solution 2

We work as above till relation (5) and we use the fact that AKAK is the perpendicular bisector of BEBE and that AMAM is the perpendicular bisector of EFEF. Since KK is the midpoint of EBEB and MM is the midpoint of EFEF, we conclude that:
KM//BF(6) KM // BF \qquad (6)
From the inscribed quadrilateral BKDLBKDL we have L^1=D^2\hat{L}_1 = \hat{D}_2. Since AF=AB\overline{AF} = \overline{AB} the angles B^1\hat{B}_1 and D^2\hat{D}_2 are equal. Therefore we conclude that L^1=B^1\hat{L}_1 = \hat{B}_1 and so
KL//BF(7) KL // BF \qquad (7)
From relations (6) and (7) we get that K,L,MK,L,M are collinear and BF//LMBF // LM.

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