SOLVED
If $A,B,C\in \mathbb{R}^2$ form a triangle and $P$ is a point in the interior then, if $X$ where the perpendicular from $P$ to $AB$ meets the triangle, and similarly for $Y$ and $Z$, then
\[\overline{PA}+\overline{PB}+\overline{PC}\geq 2(\overline{PX}+\overline{PY}+\overline{PZ}).\]
Conjectured by Erdős in 1932 (according to
[Er82e]) and proved by Mordell soon afterwards, now known as the
Erdős-Mordell inequality.
