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Geometry Difficulty 7.5 National Olympiad, round 2 Prove it Hong Kong

Let MM be the midpoint of the side BCBC of an acute ABC\triangle ABC, and let DD be the foot of perpendicular from CC to AMAM. The circumcircle of ABD\triangle ABD intersects the side BCBC again at EBE \neq B. Suppose FF is a point on the segment AEAE such that FB=FCFB = FC. Prove that FF is the midpoint of AEAE.

Solution

Let PP be the foot of perpendicular from AA to BCBC. It follows from ADC=APC=90\angle ADC = \angle APC = 90^\circ that AA, DD, PP, CC are concyclic. Considering the power of MM, we find that
ME×MB=MD×MA=MP×MC. ME \times MB = MD \times MA = MP \times MC.
Since MB=MCMB = MC, we have ME=MPME = MP. Note that FMFM is the perpendicular bisector of BCBC. Therefore, FMAPFM \parallel AP. As ME=MPME = MP, it follows from the intercept theorem that FF is the midpoint of AEAE.

Figure 1

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