$\dfrac{1}{a+b}\le \dfrac{1}{4}(\dfrac{1}{a}+\dfrac{1}{b})$
Có $\dfrac{1}{2x+y+z} \le \dfrac{1}{4}(\dfrac{1}{2x}+\dfrac{1}{y+z})\le\dfrac{1}{4}(\dfrac{1}{2x}+\dfrac{1}{4}(\dfrac{1}{y}+\dfrac{1}{z}))=\dfrac{1}{8x}+\dfrac{1}{16y}+\dfrac{1}{16z}$
Tương tự:
$\dfrac{1}{2y+z+x}\le \dfrac{1}{8y}+\dfrac{1}{16z}+\dfrac{1}{16x}$
$\dfrac{1}{2z+x+y}\le \dfrac{1}{8z}+\dfrac{1}{16x}+\dfrac{1}{16y}$
$\sum \dfrac{1}{2x+y+z} \le \dfrac{1}{4}(\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z})=\dfrac{1}{16}$
Vậy đề có vấn đề. Chắc là $\dfrac{1}{x}+\dfrac{1}{y}+\dfrac{1}{z}=4$