1. Cho $a,b,c$ thỏa mãn:
$\left\{\begin{matrix} a^2+b^2+c^2=2 & \\ ab+bc+ca=1& \end{matrix}\right.$
CMR $\frac{-4}{3}\leq a,b,c\leq \frac{4}{3}$
2. Cho $x,y,z$ thỏa mãn : $\left\{\begin{matrix} x+y+z=5 & \\ x^2+y^2+x^2=9& \end{matrix}\right.$
CMR $1\leq x,y,z\leq \frac{7}{3}$