Cho $a, b, c\geq 0$ thoả mãn $a+b+c=1$
Cmr
$\left ( \frac{1}{a}-1 \right )\left ( \frac{1}{b}-1 \right )\left ( \frac{1}{c}-1 \right )\geq 8$
Ta có :
$\prod \left ( \frac{1}{a}-1 \right )=\prod \left ( \frac{b+c}{a} \right )\geq \prod 2.\frac{\sqrt{bc}}{a}\geq8$