[ (1 + XY ) (1 + + YZ ) (1 + + ZX ) ]    1 + 1 XY + 1 + 1 YZ + 1 +...

2/. Ta cú: [ ^{(1 + xy ) (1 + + yz ) (1 + + zx )} ] ^   ₁ + ¹ _xy ⁺ ₁ + ¹ _yz ⁺ ₁ + ¹ _zx ^ ữ  ^{≥ 9} 0.25

9 9

⇔ ≥ ≥

P xy yz zx x y z

+ + + + + + 0.25

3 3

⇒ 9 3

P ≥ = 0.25

6 2

Vậy GTNN là P

= 3

2 khi x = y = z 0.25