First Problem, IMAR Competition 2012

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Laurențiu Ploscaru
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First Problem, IMAR Competition 2012

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Let $K$ be a convex planar set, symmetric about a point $O$, and let $X, Y, Z$ be three points in $K$. Show that $K$ contains the head of one of the vectors $\overrightarrow{OX}\pm \overrightarrow{OY},\ \overrightarrow{OY}\pm \overrightarrow{OZ},\ \overrightarrow{OZ}\pm \overrightarrow{OX}$.
People are strange when you're a stranger,
Faces look ugly when you're alone.
Women seem wicked when you're unwanted,
Streets are uneven when you're down.
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