BT: Cho 3 sè d¬ng a, b, c ; a +b +c = m lµ 1 h»ng sè T×m min cña A 2 2 2 a b c b c a c a b + + + + + C¸ch 1: ¸p dông B§T C«si cho 3 sè d¬ng ta cã: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 3 3 2 a b c 3 a b b c a c 0 1 1 1 1 3 0 a b b c a c a b b c c a 1 1 1 2 a b c 9 a b b c c a + + ≥ + + + > + + ≥ > + + + + + +   ⇒ + + + + ≥  ÷ + + +   a b c a b c a b c 9 a b b c a c 2 c a b 3 a b b c a c 2 + + + + + + ⇒ + + ≥ + + + ⇒ + + ≥ + + + ( ) ( ) 2 2 2 c a b 3 . a b c a b c a b b c a c 2 a b c a b c m b c c a a b 2 2 min   ⇔ + + + + ≥ + +  ÷ + + +   + + ⇔ + + ≥ = + + + ⇒ C¸ch 2: ¸p dông B§T C«si ta cã: 2 2 a b c a b c 2 a b c 4 b c 4 + + + ≥ × = + + T¬ng tù: 2 2 b c a b c a 4 c a b c a b 4 + + ≥ + + + ≥ + Céng tõng vÕ m A 2 ⇒ ≥ C¸ch 3: ¸p dông B§T Bunhia copxti ta cã: ( ) 2 2 2 2 a b c . b c c a b b c c a a b a b c . b c . c a . a b b c c a a b   + + + + + + ≥  ÷ + + +     ≥ + + + + +  ÷ + + +   ⇒ C¸ch 4: Gi¶ sö 2 2 2 a b c 0 suy ra a b c≥ ≥ > ≥ ≥ 1 1 1 b c c a a b ≥ ≥ + + + ¸p dông B§T TrªbsÐp cho 6 sè trªn ( ) 2 2 2 2 2 1 1 1 1 a b 1 a b c 3 b c c a a b b c a c a b     + + + + ≤ + +  ÷  ÷ + + + + + +     ( ) ( ) ( ) 2 2 2 2 1 a b c 1 1 1 1 a b c . b c a c a b 9 b c c a a b 1 m a b c 9 Theo C 18 2   ⇒ + + ≥ + + + +  ÷ + + + + + =   ≥ + + = . + +  ÷ + + +   ⇒ C¸ch 4: Gi¶ sö 2 2 2 a b c 0 suy ra a b c≥ ≥ > ≥ ≥ 1 1 1 b c c a a b ≥ ≥ + + + ¸p dông B§T TrªbsÐp cho 6 sè trªn ( ) 2 2 2 2 2 1 1 1 1 a b 1 a b c 3 b c c a a b b c.  + + + + ≤ + +  ÷  ÷ + + + + + +     ( ) ( ) ( ) 2 2 2 2 1 a b c 1 1 1 1 a b c . b c a c a b 9 b c c a a b 1 m a b c 9 Theo C 18 2   ⇒ + + ≥ + + + +  ÷ + + + + + =   ≥ + + = . +c = m lµ 1 h»ng sè T×m min cña A 2 2 2 a b c b c a c a b + + + + + C¸ch 1: ¸p dông B§T C«si cho 3 sè d¬ng ta cã: ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) 3 3 2 a b c 3 a b b c a c 0 1 1 1 1 3 0 a b