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Geometry Difficulty 6.5 National olympiad Prove it Greece

Let ABCABC be an acute angle scalene triangle with AB<ACAB < AC, inscribed in the circle c(O,R)c(O, R). The circle c1(B,AB)c_1(B, AB) intersects the side ACAC at point KK and the circle cc at point EE. The line KEKE intersects the circle cc also at point FF. The line BOBO intersects KEKE at point LL and ACAC at point MM. Finally, the line AEAE intersects BFBF at point DD. Prove that the polygons DLMFDLMF and BDKMEBDKME are cyclic.

Solution

In the circle cc, the chords ABAB and BEBE are equal and hence
A^1=F^1=F^2=E^1=C^, \hat{A}_1 = \hat{F}_1 = \hat{F}_2 = \hat{E}_1 = \hat{C},
Figure 1
Figure 1
because in the circle cc the above angles go to the equal arches ABAB and BEBE. OBOB is the line of the centers of the circles cc and c1c_1, and so it is perpendicular bisector of the common chord AEAE, i.e. ADBMAD \perp BM. We will prove that BFACBF \perp AC, whence point DD will be the orthocenter of the triangle ABMABM.

Moreover in the triangle BKFBKF we have: K^1=F^2+B^2\hat{K}_1 = \hat{F}_2 + \hat{B}_2. (1)

The triangle BKEBKE is isosceles (BE=BKBE = BK, as radii of the circle c1c_1), whence: K^1=E^3=E^1+E^2\hat{K}_1 = \hat{E}_3 = \hat{E}_1 + \hat{E}_2. (2)

From the cyclic quadrilateral ABEFABEF we have:
B^1=E^2\hat{B}_1 = \hat{E}_2 (3)

From relations (1), (2) and (3) (taking in mind equalities F^2=E^1=C^\hat{F}_2 = \hat{E}_1 = \hat{C}) we find: B^1=B^2\hat{B}_1 = \hat{B}_2 (4).

From the equalities B^1=B^2\hat{B}_1 = \hat{B}_2 and F^1=F^2=C^\hat{F}_1 = \hat{F}_2 = \hat{C}, we conclude that ABFABF and KBFKBF are equal. Hence BFBF is the perpendicular bisector of AKAK. Since ADBMAD \perp BM, we conclude that DD is the orthocenter of the triangle ABMABM. Therefore we have M^1=A^1=90MBA\hat{M}_1 = \hat{A}_1 = 90^\circ - MBA and in combination with A^1=F^2=C^\hat{A}_1 = \hat{F}_2 = \hat{C} we have that M^1=F^2\hat{M}_1 = \hat{F}_2, whence the quadrilateral DLMFDLMF is cyclic.

Since B^2=E^2\hat{B}_2 = \hat{E}_2, the quadrilateral DKEBDKEB is cyclic and since E^1=M^1\hat{E}_1 = \hat{M}_1, the quadrilateral DBEMDBEM is cyclic. Therefore the polygon BDKMEBDKME is cyclic.

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