A= $\frac{xy + yz + xz}{xyz}$ = $\frac{1}{x}$ + $\frac{1}{y}$ + $\frac{1}{z}$
Do |x| ≥ 3 => x² ≥ 9 => $\frac{1}{x^{2} }$ ≤ $\frac{1}{9}$
Tương tự $\frac{1}{y^{2} }$ ≤ $\frac{1}{9}$ ; $\frac{1}{z^{2} }$ ≤ $\frac{1}{9}$
A²= ($\frac{1}{x}$ + $\frac{1}{y}$ +c$\frac{1}{z}$ ) ≤ 3.($\frac{1}{x^{2} }$+$\frac{1}{y^{2} }$+$\frac{1}{z^{2} }$)
=3.($\frac{1}{9}$ .$\frac{1}{9}$ .$\frac{1}{9}$ )
=> A²≤ $\frac{1}{3}$ => A≤ 1
