KöMaL 2006 A400

Prove that $\frac{x}{y+2z+3u}+ \frac{y}{z+2u+3x}+ \frac{z}{u+2x+3y} +\frac{u}{x+2y+3z} \ge \frac{2}{3}.$ for all real numbers $x$ , $y$ , $z$ , $u$ .

Official text KöMaL. Quoted with source.

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