Bài 144(Poland MO): Cho a,b,c>0. CMR: nếu ab+bc+ca=1 thì
$\frac{(a^2+1)^2}{bc(b+c)}+\frac{(b^2+1)^2}{ca(c+a)}+\frac{(c^2+1)^2}{ab(a+b)}\geq 8(\sqrt{ab}+\sqrt{bc}+\sqrt{ca})$
Ta có : $a^2+1=a^2+ab+bc+ac=(a+b)(a+c)$
Theo Cosi và BDT $(x+y)(y+z)(x+z)\geq \frac{8(\sum x)(\sum xy)}{9}$, $(x+y)^3\geq 4xy(x+y)$ ta có :
$\sum \frac{(a^2+1)^2}{bc(b+c)}=\sum \frac{\left [ (a+b)(a+c) \right ]^2}{bc(b+c)}\geq 3\sqrt[3]{\frac{(a+b)^4(b+c)^4(c+a)^4}{(abc)^2(a+b)(b+c)(c+a)}}=3\sqrt[3]{\frac{(a+b)^3(b+c)^3(c+a)^3}{(abc)^2}}\geq 3\sqrt[3]{\frac{4ab(a+b).4bc(b+c).4ac(a+c)}{(abc)^2}}=3\sqrt[3]{64(a+b)(b+c)(c+a)}\geq 12\sqrt[3]{\frac{8(\sum a)(\sum ab)}{9}}=12\sqrt[3]{\frac{8(\sum a).1}{9}}\geq 12\sqrt[3]{\frac{8\sqrt{3(\sum ab)}}{9}}=12\sqrt[3]{\frac{8\sqrt{3}}{9}}=24\sqrt[3]{\frac{1}{\sqrt{27}}}=\frac{24}{\sqrt{3}}=8\sqrt{3}= > \sum \frac{(a^2+1)^2}{bc(b+c)}\geq 8\sqrt{3}$ (1)
Theo Bunhia có :$8(\sum \sqrt{ab})\leq 8\sqrt{3(\sum ab)}=8\sqrt{3.1}=8\sqrt{3}$ (2)
Từ (1),(2) $= > \sum \frac{(a^2+1)^2}{bc(b+c)}\geq 8(\sum \sqrt{ab})$
Dấu = xảy ra khi $a=b=c=\frac{1}{\sqrt{3}}$