Question Number 177068 by LOSER last updated on 30/Sep/22
$${Let}\:{a},{b},{c}\:{be}\:{real}\:{numbers}\:{such}\:{that}: \\ $$$${a}+{b}+{c}=\mathrm{0}.\:{Prove}\:{that}: \\ $$$$\frac{{a}^{\mathrm{2}} {b}^{\mathrm{2}} {c}^{\mathrm{2}} }{\mathrm{4}}+\frac{\left({ab}+{bc}+{ca}\right)^{\mathrm{3}} }{\mathrm{27}}\leqslant\mathrm{0} \\ $$
Answered by Frix last updated on 30/Sep/22
$${c}=−{a}−{b} \\ $$$$\frac{{a}^{\mathrm{2}} {b}^{\mathrm{2}} {c}^{\mathrm{2}} }{\mathrm{4}}+\frac{\left({ab}+{bc}+{ca}\right)^{\mathrm{3}} }{\mathrm{27}}= \\ $$$$=−\frac{\left({a}−{b}\right)^{\mathrm{2}} \left({a}+\mathrm{2}{b}\right)^{\mathrm{2}} \left(\mathrm{2}{a}+{b}\right)^{\mathrm{2}} }{\mathrm{108}}\leqslant\mathrm{0}\:\mathrm{obviously}\:\mathrm{true} \\ $$
Commented by Frix last updated on 30/Sep/22
$$\mathrm{how}\:\mathrm{I}\:\mathrm{got}\:\mathrm{this}: \\ $$$${c}=−{a}−{b}\wedge{b}={pa}\:\Rightarrow\:{c}=−{a}\left(\mathrm{1}+{p}\right) \\ $$$$\frac{{a}^{\mathrm{2}} {b}^{\mathrm{2}} {c}^{\mathrm{2}} }{\mathrm{4}}+\frac{\left({ab}+{bc}+{ca}\right)^{\mathrm{3}} }{\mathrm{27}}= \\ $$$$=−\frac{{a}^{\mathrm{6}} \left({p}^{\mathrm{6}} +\mathrm{3}{p}^{\mathrm{5}} −\frac{\mathrm{3}{p}^{\mathrm{4}} }{\mathrm{4}}−\frac{\mathrm{13}{p}^{\mathrm{3}} }{\mathrm{2}}−\frac{\mathrm{3}{p}^{\mathrm{2}} }{\mathrm{4}}+\mathrm{3}{p}+\mathrm{1}\right)}{\mathrm{27}} \\ $$$$\mathrm{factors}\:\mathrm{of}\:\mathrm{the}\:\mathrm{polynome} \\ $$$${p}^{\mathrm{6}} +\mathrm{3}{p}^{\mathrm{5}} −\frac{\mathrm{3}{p}^{\mathrm{4}} }{\mathrm{4}}−\frac{\mathrm{13}{p}^{\mathrm{3}} }{\mathrm{2}}−\frac{\mathrm{3}{p}^{\mathrm{2}} }{\mathrm{4}}+\mathrm{3}{p}+\mathrm{1} \\ $$$${p}={q}−\frac{\mathrm{1}}{\mathrm{2}} \\ $$$${q}^{\mathrm{6}} −\frac{\mathrm{9}{q}^{\mathrm{4}} }{\mathrm{2}}+\frac{\mathrm{81}{q}^{\mathrm{2}} }{\mathrm{16}} \\ $$$${q}^{\mathrm{2}} \left({q}^{\mathrm{2}} −\frac{\mathrm{9}}{\mathrm{4}}\right)^{\mathrm{2}} \\ $$$${q}^{\mathrm{2}} \left({q}−\frac{\mathrm{3}}{\mathrm{2}}\right)^{\mathrm{2}} \left({q}+\frac{\mathrm{3}}{\mathrm{2}}\right)^{\mathrm{2}} \\ $$$${q}={p}+\frac{\mathrm{1}}{\mathrm{2}} \\ $$$$\left({p}−\mathrm{1}\right)^{\mathrm{2}} \left({p}+\mathrm{2}\right)^{\mathrm{2}} \left({p}+\frac{\mathrm{1}}{\mathrm{2}}\right)^{\mathrm{2}} \\ $$$${p}=\frac{{b}}{{a}} \\ $$$$\frac{\left({a}−{b}\right)^{\mathrm{2}} \left({a}+\mathrm{2}{b}\right)^{\mathrm{2}} \left(\mathrm{2}{a}+{b}\right)^{\mathrm{2}} }{\mathrm{4}{a}^{\mathrm{6}} } \\ $$
Commented by LOSER last updated on 01/Oct/22
$${You}'{re}\:{too}\:{strong},\:{sir}! \\ $$