4 cach giai 1 bai toan nho

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
 
⇒ + + ≥ + + + +
 ÷
+ + + + + =
 
≥ + + =