Cho các số a, b, c dương. Chứng minh
$\sqrt{\left ( a^{2}b+b^{2}c+c^{2}a \right )\left ( ab^{2}+bc^{2}+ca^{2} \right )}\geq abc+\sqrt[3]{\left ( a^{3}+abc \right )\left ( b^{3}+abc \right )\left ( c^{3}+abc \right )}$
$BDT\Leftrightarrow \sqrt{\left ( \frac{a}{c}+\frac{c}{b}+\frac{b}{a} \right )\left ( \frac{b}{c}+\frac{c}{a}+\frac{a}{b} \right )}\geq 1+\sqrt[3]{\prod \left ( \frac{a^{2}}{bc}+1 \right )}$ $(1)$
Đặt $\left\{\begin{matrix} x=\frac{a}{b}\\ y=\frac{b}{c} \\ z=\frac{c}{a} \end{matrix}\right.$ với $xyz=1$
$(1)$ trở thành $\sqrt{(x+y+z)(xy+yz+zx)}\geq 1+\sqrt[3]{\prod (1+\frac{x}{z})}$
$\Leftrightarrow \sqrt{(x+y)(y+z)(z+x)+xyz}\geq 1+\sqrt[3]{\frac{(x+y)(y+z)(z+x)}{xyz}}$
$\Leftrightarrow \sqrt{(x+y)(y+z)(z+x)+1}\geq 1+\sqrt[3]{(x+y)(y+z)(z+x)}$
$\Leftrightarrow \sqrt[3]{\prod (x+y)}\left ( \sqrt[3]{\prod \left ( x+y \right )} -2\right )\left ( \sqrt[3]{\prod (x+y)}+1 \right )\geq 0$ đúng