(A B C+ + ) (A2+B2+C2)≥(A B C+ + )(AB BC+ +CA)≥93ABC A B C3 2 2 2 =9ABC
Bài41:Lời Giải: Theo AM-GM ta có:
(
a b c+ +) (
a2
+b2
+c2
)
≥(
a b c+ +)(
ab bc+ +ca)
≥93
abc a b c3
2
2
2
=9abc.∑ ∑
+ + +a b c a ab+ +2ab bc ca= + =P a b c abcKhi ñó:( ) (
2
)
2
2
2
∑ ∑ ∑ ∑ ∑ ∑
+ + 2
2
8 2a a a a a abab bc ca= + + + + +9 9a b c abc abc abc2
a ab a a a ab≥ + + ≥ + + =2 2 8 18 28.abc abc abcQ.