BĐT $\frac{x^{3}+y^{3}+z^{3}-3xyz}{3}\geq \frac{3}{4}.\begin{vmatrix} (a-b)(b-c)(c-a) \end{vmatrix}$Bài 426 . Cho các số thực không âm $a,b,c$ . Chứng minh rằng $\frac{a^{3}+b^{3}+c^{3}}{3}\geq abc+\frac{3}{4}\begin{vmatrix} (a-b)(b-c)(c-a) \end{vmatrix}$
$\Leftrightarrow \frac{(a+b+c).\sum (a-b)^{2}}{6}\geq \frac{3}{4}.\begin{vmatrix} (a-b)(b-c)(c-a) \end{vmatrix}$
Ta có :
$2(a+b+c)= \begin{vmatrix} a+b \end{vmatrix}+\begin{vmatrix} b+c\end{vmatrix}+\begin{vmatrix} c+a\end{vmatrix}\geq \begin{vmatrix} a-b \end{vmatrix}+\begin{vmatrix} b-c \end{vmatrix}+\begin{vmatrix} c-a\end{vmatrix}$
$\geq 3\sqrt[3]{\begin{vmatrix} a-b \end{vmatrix}\begin{vmatrix} b-c\end{vmatrix}\begin{vmatrix} c-a \end{vmatrix}}$
$\sum (a-b)^{2}\geq 3\sqrt[3]{(\begin{vmatrix} a-b \end{vmatrix}\begin{vmatrix} b-c \end{vmatrix}\begin{vmatrix} c-a \end{vmatrix})^{2}}$
$\Rightarrow VT= \frac{2(a+b+c)\sum (a-b)^{2}}{12}\geq \frac{9}{12}.\begin{vmatrix} a-b \end{vmatrix}\begin{vmatrix} b-c \end{vmatrix}\begin{vmatrix} c-a \end{vmatrix}= \frac{3}{4}.\begin{vmatrix} a-b \end{vmatrix}\begin{vmatrix} b-c \end{vmatrix}\begin{vmatrix} c-a \end{vmatrix}= VP$