2. gt $\Leftrightarrow (x-\frac{1}{2})^{2} + (y-\frac{1}{2})^{2} +(z-\frac{1}{2})^{2}=\frac{25}{12}$
$(x-\frac{1}{2}+y-\frac{1}{2}+z -\frac{1}{2})^{2} \leq 3[(x-\frac{1}{2})^{2} +(y-\frac{1}{2})^{2}+(z-\frac{1}{2})^{2}]=\frac{25}{4}$
$\Leftrightarrow \frac{-5}{2}\leq x+y+z-\frac{3}{2}\leq \frac{5}{2}$
$\Rightarrow x+y+z\geq -1$
dấu "=" $\Leftrightarrow x=y=z=\frac{-1}{3}$