a) \(\dfrac{1}{a}\) + \(\dfrac{1}{b}\) + \(\dfrac{1}{c}\)≥\(\dfrac{9}{a+b+c}\)
<=> ( \(\dfrac{1}{a}\)+ \(\dfrac{1}{b}\) + \(\dfrac{1}{c}\))(a+b+c) ≥ 9
Ta có : \(\dfrac{1}{a}\) + \(\dfrac{1}{b}\) + \(\dfrac{1}{c}\) ≥ 3.căn bậc 3 1/abc(Cô-si)
a+b+c ≥ 3 căn bậc 3 abc
(1/a + 1/b + 1/c)(a+c+c) ≥ 9 căn bậc 3 abc/abc = 9
<=> 1/a + 1/b + 1/c ≥ 9(a+b+c)
Dấu ''='' xảy ra khi : a=b =c
Cách khác :
Áp dụng BĐT Cauchy dạng Engel , ta có :
\(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\ge\dfrac{\left(1+1+1\right)^2}{a+b+c}=\dfrac{9}{a+b+c}\)
\("="\Leftrightarrow a=b=c\)
