Cho $a, b, c > 0$ thoả mãn $(a + b)(b + c)(c + a) = 8$. Tìm GTNN của :
$P = \frac{1}{\sqrt[3]{abc}} + \frac{1}{a + 2b} + \frac{1}{b + 2c} + \frac{1}{c + 2a}$
$(2a+2b)(2b+2c)(2c+2a)=64\Leftrightarrow 64\geq \left ( \sqrt[3]{abc}+\sqrt[3]{(a+2b)(b+2c)(c+2a)} \right )^{3}\Rightarrow \sqrt[3]{abc}+\sqrt[3]{(a+2b)(b+2c)(c+2a)}\leq 4$
Vậy $P\geq \frac{1}{\sqrt[3]{abc}}+\frac{3}{\sqrt[3]{(a+2b)(b+2c)(c+2a)}}= \frac{\frac{1}{3}}{\sqrt[3]{abc}}+\frac{3}{\sqrt[3]{(a+2b)(b+2c)(c+2a)}}+\frac{\frac{2}{3}}{\sqrt[3]{abc}}\geq \frac{\left ( \sqrt{\frac{1}{3}}+\sqrt{3} \right )^{2}}{4}+\frac{\frac{2}{3}}{1}=2$