Ta có:
\(\dfrac{ab+bc+ca}{2\left(a^2+b^2+c^2\right)}+\dfrac{1}{6}\left(\dfrac{a^2+b^2+c^2}{abc}\right)\ge2\sqrt{\dfrac{1}{12}\left(\dfrac{ab+ca+ca}{abc}\right)}=\sqrt{3\left(\dfrac{ab+bc+ca}{abc}\right)}\)
Nên ta chỉ cần cm:
\(\sqrt{\dfrac{1}{3}\left(\dfrac{ab+bc+ca}{abc}\right)}\ge\dfrac{a+b+c}{3}\Leftrightarrow3\left(\dfrac{ab+bc+ca}{abc}\right)\ge\left(a+b+c\right)^2\)
Thật vậy, ta có:
\(\dfrac{3\left(ab+bc+ca\right)}{abc}=\dfrac{\left(a^2b+b^2c+c^2a\right)\left(ab+bc+ca\right)}{abc}\)
\(=\left(\dfrac{a}{c}+\dfrac{b}{a}+\dfrac{c}{b}\right)\left(ac+ab+bc\right)\ge\left(a+b+c\right)^2\) (Bunhiacopxki)
Dấu "=" xảy ra khi \(a=b=c=1\)
