A = (X + Y)(X + Z) = X2 + XZ + XY + YZ = X(X + Y + Z) + YZ ≥2 XYZ...

136. A = (x + y)(x + z) = x

²

+ xz + xy + yz = x(x + y + z) + yz

≥

2 xyz(x y z) 2

+ +

=

min A = 2 khi chẳng hạn y = z = 1 , x =

2

^{- 1.}