Maths Olympiad Prep

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Geometry Difficulty 6.3 National olympiad Prove it

In triangle ABCA B C, AD(DBC)A D (D \in B C) is a median, EE is a point on ACA C, and FF is the intersection of BEB E with ADA D.

Prove: If BFFE=BCAB+1\frac{B F}{F E}=\frac{B C}{A B}+1, then BEB E is an angle bisector.

Solution

Let B,C,EB^{\prime}, C^{\prime}, E^{\prime} be the projections of B,CB, C, and EE onto the line ADA D. The right triangles DBB\triangle D B B^{\prime} and DCC\triangle D C C^{\prime} are congruent, since DD is the midpoint of BCB C and the acute angles at DD are equal. This implies that BB=CCB B^{\prime}=C C^{\prime}.
In the triangle ACC\triangle A C^{\prime} C, EEE E^{\prime} is parallel to CCC C^{\prime}, and thus

ACAE=CCEE=BBEE. \frac{A C}{A E}=\frac{C C^{\prime}}{E E^{\prime}}=\frac{B B^{\prime}}{E E^{\prime}}.

The right triangles BBF\triangle B B^{\prime} F and EEF\triangle E E^{\prime} F are similar, as the angles at FF are equal. This implies BBEE=BFFE\frac{B B^{\prime}}{E E^{\prime}}=\frac{B F}{F E}, which, together with the previous relationship, leads to ACAE=BFEF\frac{A C}{A E}=\frac{B F}{E F}.
Replacing BFEF\frac{B F}{E F} in the initial relationship with ACAE\frac{A C}{A E}, we get:
BCAB+1=ACAE=BC+ABAB\frac{B C}{A B}+1=\frac{A C}{A E}=\frac{B C+A B}{A B}, which simplifies to ACAEAE=BCAB\frac{A C-A E}{A E}=\frac{B C}{A B} and finally to ECAE=BCAB\frac{E C}{A E}=\frac{B C}{A B}.
According to the converse of the Angle Bisector Theorem, BE=wβB E=w_{\beta}.

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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.