Maths Olympiad Prep

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Geometry Difficulty 5.6 AIME, harder Prove it

Example 13. If the diagonals ADA D, BEB E, CFC F of a convex hexagon ABCDEFA B C D E F all bisect the area of the hexagon. Prove: ADA D, BEB E, CFC F are concurrent.
(1965(1965, Polish Mathematical Olympiad)

Solution

Prove that SABCD=12SABCDEF=SBCDES_{A B C D}=\frac{1}{2} S_{A B C D E F}=S_{B C D E},
and SABCD=SABD+SDBCS_{A B C D}=S_{\triangle A B D}+S_{\triangle D B C},
and SBCDE=SEBD+SDBCS_{B C D E}=S_{\triangle E B D}+S_{\triangle D B C},
thus SABD=SEBDS_{\triangle A B D}=S_{\triangle E B D}.
Since they share the same base, then AE//BDA E / / B D.
Similarly, we can prove that AC//DF,CE//BFA C / / D F, C E / / B F.
Assume AD,BEA D, B E intersect at MM. Consider the homothety centered at MM that maps AA to DD. Since AE//BDA E / / B D, BB is the image of EE under this transformation.

The image of ACA C is a line through DD and parallel to it, and the image of ECE C is a line through BB and parallel to it, so FF is the common point of these two image lines, thus FF is the image of point CC.
Therefore, CFC F passes through the homothety center MM.

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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.