$a+\,b+\,c+\,d=\,1$, chứng minh rằng:
\[3\,\geq\, (\,1-ad\,)\,(\,1-bc\,)+(\,1-ca\,)\,(\,1-bd\,)+(\,1-ab\,)\,(\,1-cd\,)\,\geq \,\frac{675}{256}\]
$a+\,b+\,c+\,d=\,1$, chứng minh rằng:
\[3\,\geq\, (\,1-ad\,)\,(\,1-bc\,)+(\,1-ca\,)\,(\,1-bd\,)+(\,1-ab\,)\,(\,1-cd\,)\,\geq \,\frac{675}{256}\]
$$\frac{8}{243}\,(90+ 37\sqrt{3})\geqq(1-ab)\,(1-bc)\,(1-cd)\,(1-da)\,(1-bd)\,(1-ac)\geqq \frac{15^{\,6}}{16^{\,6}}$$
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