Chứng minh rằng với $ \it{a}+ \it{b}\geqq \it{0},\,\it{b}+ \it{c}\geqq \it{0},\,\it{c}+ \it{a}\geqq \it{0} $$,$ không thể tồn tại $ \it{k}= \it{constant} $$ : $
$$\sum\limits_{cyc}\,\it{a}^{\,\it{3}}- \sum\limits_{cyc}\,\it{a}^{\,\it{2}}\it{b}\geqq \it{k}\it{(}\,\,\it{a}- \it{b}\,\,\it{)}\it{(}\,\,\it{a}- \it{c}\,\,\it{)}\it{(}\,\,\it{b}+ \it{c}\,\,\it{)}$$