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Geometry Difficulty 5.0 AIME, harder Prove it Belarus

The bisector of the angle CABCAB of triangle ABCABC intersects the side CBCB at LL. The point DD is the foot of the perpendicular from CC to ALAL and the point EE is the foot of the perpendicular from LL to ABAB. The lines CBCB and DEDE meet at FF.
Prove that AFAF is an altitude of the triangle ABCABC.

Solutions — 2

Solution 1

Let FF' be the foot of an altitude from AA in the triangle ABCABC.

Figure 1

It is enough to prove that the points DD, FF' and EE are colinear, since it will lead to F=FF = F' which means that AFAF is an altitude of the triangle ABCABC. Since CDA=CFA=90\angle CDA = \angle CF'A = 90^\circ, the quadrilateral CDFACDF'A is cyclic and therefore CFD=CAD\angle CF'D = \angle CAD. Since AFL=AEL=90\angle AF'L = \angle AEL = 90^\circ, the quadrilateral AEFLAEF'L is cyclic and hence BEF=LAE\angle BEF' = \angle LAE. Since CAL=LAB\angle CAL = \angle LAB, then CFD=BFE\angle CF'D = \angle BF'E which means that DD, FF' and EE are colinear.

Solution 2

Let the lines ELEL and CDCD meet at KK. Since the angles ADKADK and AEKAEK are right, the points AA, KK, DD and EE lie on the circle with diameter AKAK. Hence DKE=DAE\angle DKE = \angle DAE. Since ADAD is the bisector, DAE=DAC\angle DAE = \angle DAC, whence DKE=DAC\angle DKE = \angle DAC. The latter is equivalent to DKL=CAL\angle DKL = \angle CAL, whence AA, LL, CC and KK lie on a circle.
Consider the Simson line of AA and the triangle LCKLCK. The foot of the perpendicular from AA to CKCK is DD, and to LKLK is EE, therefore this line is DEDE. Since DEDE meet CLCL at FF, FF is the foot of the perpendicular from AA to CLCL.

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