Cho $\it{3}$ số thực $\it{a},\,\it{b},\,\it{c}$ thỏa $\it{:}$ $\it{ab}+ \it{bc}+ \it{ca}= \it{4}$ $\it{.}$ Chứng minh $\it{:}$
$$\it{3}\sum \it{a}^{\,\it{2}}- \it{4}\sum \it{a}^{\,\it{4}}\leqq -\,\frac{\it{28}}{\it{3}}$$
$\lceil$ B*W $\it{!}$ $\rfloor$ $\lceil$ S*O*S $\it{!}$ $\rfloor$
Spoiler