M = ( A + B ) (2 ≤ A + B ) (2+ A − B )2 = 2A 2B 2 + ≤

81. Ta cĩ : M = ( a + b ) (

2

a + b ) (

2

+ a b )

2

= 2a 2b 2 + .

 =

= ⇔   ⇔ = =

1

a b

max M 2 a b

a b 1 2

 + = .