Ta có: bc(a2+1) = (a+b)(a+c)
\(\Rightarrow\) \(\dfrac{a}{\sqrt{bc\left(1+a^2\right)}}\) =\(\sqrt{\dfrac{a}{a+b}}.\sqrt{\dfrac{a}{a+c}}\)
Áp dụng BĐT Cô-si: \(\sqrt{\dfrac{a}{a+b}}.\sqrt{\dfrac{a}{a+c}}\) \(\le\) \(\dfrac{1}{2}\left(\dfrac{a}{b+c}+\dfrac{a}{a+c}\right)\)
\(\Rightarrow\) \(\dfrac{a}{\sqrt{bc\left(1+a^2\right)}}\) \(\le\) \(\dfrac{1}{2}\left(\dfrac{a}{a+b}+\dfrac{a}{a+c}\right)\)
CMTT: \(\dfrac{b}{\sqrt{ac\left(1+b^2\right)}}\) \(\le\) \(\dfrac{1}{2}\left(\dfrac{b}{a+b}+\dfrac{b}{a+c}\right)\)
\(\dfrac{c}{\sqrt{ab\left(1+c^2\right)}}\) \(\le\) \(\dfrac{1}{2}\left(\dfrac{c}{a+c}+\dfrac{c}{c+b}\right)\)
\(\Rightarrow\) S \(\le\) \(\dfrac{1}{2}\left(\dfrac{a}{b+a}+\dfrac{a}{c+a}+\dfrac{b}{a+b}+\dfrac{b}{c+b}+\dfrac{c}{a+c}+\dfrac{c}{b+c}\right)\)
\(\Rightarrow\) S\(\le\) \(\dfrac{1}{2}.3=\dfrac{3}{2}\)
Vậy Smax = \(\dfrac{3}{2}\)
Dấu "=" xảy ra\(\Leftrightarrow\) \(\left\{{}\begin{matrix}a=b=c\\a+b+c=abc\end{matrix}\right.\)
\(\Leftrightarrow\) \(a=b=c=\sqrt{3}\)
