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V

vansang02121998

Áp dụng Cauchy

$4(1-x)(1-z) \le (2-x-z)^2=(1+y)^2$

$\Rightarrow 4(1-x)(1-y)(1-z) \le (1-y)(1+y)^2=(1-y^2)(1+y) \le 1+y=x+2y+z$

Vậy, $x+2y+z \ge 4(1-x)(1-y)(1-z)$

Dấu $"="$ xảy ra khi $x=z=0,5;y=0$
 
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