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Geometry Difficulty 7.0 National olympiad, round 2 Prove it

Let ABCA B C be a triangle, (BC<AB)(B C < A B). The line \ell passing through the vertices CC and orthogonal to the angle bisector BEB E of B\angle B, meets BEB E and the median BDB D of the side ACA C at points FF and GG, respectively. Prove that segment DFD F bisects the segment EGE G.

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Solution

Let CFAB={K}C F \cap A B=\{K\} and DFBC={M}D F \cap B C=\{M\}. Since BFKCB F \perp K C and BFB F is angle bisector of KBC\angle K B C, we have that KBC\triangle K B C is isosceles i.e. BK=BCB K=B C, also FF is midpoint of KCK C. Hence DFD F is midline for ACK\triangle A C K i.e. DFAKD F \| A K, from where it is clear that MM is a midpoint of BCB C.

We will prove that GEBCG E \| B C. It is sufficient to show BGGD=CEED\frac{B G}{G D}=\frac{C E}{E D}. From DFAKD F \| A K and DF=AK2D F=\frac{A K}{2} we have

BGGD=BKDF=2BKAK \frac{B G}{G D}=\frac{B K}{D F}=\frac{2 B K}{A K}

Also

CEDE=CDDEDE=CDDE1=ADDE1=AEDEDE1=AEDE2==ABDF2=AK+BKAK22=2+2BKAK2=2BKAK \begin{gathered} \frac{C E}{D E}=\frac{C D-D E}{D E}=\frac{C D}{D E}-1=\frac{A D}{D E}-1=\frac{A E-D E}{D E}-1=\frac{A E}{D E}-2= \\ =\frac{A B}{D F}-2=\frac{A K+B K}{\frac{A K}{2}}-2=2+2 \frac{B K}{A K}-2=\frac{2 B K}{A K} \end{gathered}

From (1) and (2) we have BGGD=CEED\frac{B G}{G D}=\frac{C E}{E D}, so GEBCG E \| B C, as MM is the midpoint of BCB C, it follows that the segment DFD F, bisects the segment GEG E.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.