[生活] 老封：平面几何热线

引用:

原帖由 老封 于 2007-5-16 14:09 发表
Let $ABC$ be a triangle, with incenter $I$. Its incircle touching the sides $BC, CA, AB$ at $D, E, F$, respectively. Line $EF$ intersects lines $BI$ and $CI$ at $Q$ and $P$, respectively. Lines $BP$ and $CQ$ meet at $X$. Then $XI$ is perpendicular to $BC$.