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Problem 1562

AIME late
Geometry Difficulty 5.7 Prove it Shortlist JBMO · JBMO · 2008

Let ABCABC be a triangle, (BC<AB)(BC < AB). The line ℓ\ell passing through the vertex CC and orthogonal to the angle bisector BEBE of ∠B\angle B, meets BEBE and the median BDBD of the side ACAC at points FF and GG, respectively. Prove that segment DFDF bisects the segment EGEG.
Figure 1

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

Solution:
Let CF∩AB={K}CF \cap AB = \{K\} and DF∩BC={M}DF \cap BC = \{M\}. Since BF⊥KCBF \perp KC and BFBF is angle bisector of KBC\text{KBC}, we have that △KBC\triangle KBC is isosceles, i.e. BK=BCBK = BC, also FF is midpoint of KCKC. Hence DFDF is midline for △ACK\triangle ACK, i.e. DF∥AKDF \parallel AK, from where it is clear that MM is a midpoint of BCBC.

We will prove that GE∥BCGE \parallel BC. It is sufficient to show BGGD=CEED\frac{BG}{GD} = \frac{CE}{ED}. From DF∥AKDF \parallel AK and DF=AK2DF = \frac{AK}{2} we have
BGGD=BKDF=2BKAK \frac{BG}{GD} = \frac{BK}{DF} = \frac{2BK}{AK}
Also
CEDE=CD−DEDE=CDDE−1=ADDE−1=AE−DEDE−1=AEDE−2==ABDF−2=AK+BKAK2−2=2+2BKAK−2=2BKAK \begin{gathered} \frac{CE}{DE} = \frac{CD - DE}{DE} = \frac{CD}{DE} - 1 = \frac{AD}{DE} - 1 = \frac{AE - DE}{DE} - 1 = \frac{AE}{DE} - 2 = \\ = \frac{AB}{DF} - 2 = \frac{AK + BK}{\frac{AK}{2}} - 2 = 2 + 2\frac{BK}{AK} - 2 = \frac{2BK}{AK} \end{gathered}
From (1) and (2) we have BGGD=CEED\frac{BG}{GD} = \frac{CE}{ED}, so GE∥BCGE \parallel BC, as MM is the midpoint of BCBC, it follows that the segment DFDF bisects the segment GEGE.

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