Let △ABC be a scalene triangle with AB<AC, with circumcircle c(O,R). The circle c1(A,AB) intersects the side BC at E and the circle c at F. The line EF meets for a second time the circle c at point D and the side AC at point M. The line AD intersects the side BC at point K. Finally, the circumcircle of the triangle BKD intersects AB at L. Prove that the points K,L,M lie on a line parallel to the line BF.
Solutions — 2
Solution 1
The angle F^1 is inscribed into the circle c1 with corresponding central angle BAE^. Hence: Figure 4 --- F^1=2BAE^=2A^1+A^2(1) From the cyclic quadrilateral AFDB we have: F^1=A^1(2) From (1) and (2) we get that A^1=A^2, that is AK is the bisector of the angle BAE^. Since AB=AE, we conclude that AK and hence DK is perpendicular to BC, that is DK⊥BC(3) In the circle c, cords AB and AF are equal, as radii of circle c1 and so D^1=C^. Hence the quadrilateral DKMC is inscribable. Therefore we have DK^C=DM^C=90∘, that is: DM⊥AC(4) From the inscribed quadrilateral BKDL we have BK^D=BL^D=90∘, that is: DL⊥AB(5) From the relations (3), (4) and (5) we conclude that the points K,L,M are on the Simson's of the triangle ABC corresponding to the point D. From the inscribable quadrilateral DKMC we get that M^1=C^1. Also from the inscribed quadrilateral ABDC we have C^1=A^1 and finally from the inscribed quadrilateral ABDF we have A^1=F^1. Hence M^1=F^1 from which we get that BF//LM.
Solution 2
We work as above till relation (5) and we use the fact that AK is the perpendicular bisector of BE and that AM is the perpendicular bisector of EF. Since K is the midpoint of EB and M is the midpoint of EF, we conclude that: KM//BF(6) From the inscribed quadrilateral BKDL we have L^1=D^2. Since AF=AB the angles B^1 and D^2 are equal. Therefore we conclude that L^1=B^1 and so KL//BF(7) From relations (6) and (7) we get that K,L,M are collinear and BF//LM.
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