bahieutrinh

$ 6P=\sum \ \frac{6}{a^2 + b^2} - \frac{a^3 + b^3 + c^3}{2abc} $
$  = 3 + \sum \frac{c^2}{a^2+b^2} - \frac{a^3 + b^3 + c^3}{2abc} $
$  \leq  3 + \sum \frac{c^2}{2ab} - \frac{a^3 + b^3 + c^3}{2abc} $
$  = 3 + \frac{a^3 + b^3 + c^3}{2abc} - \frac{a^3 + b^3 + c^3}{2abc}
=3 $
$ => P \leq \frac{1}{2} Dau = xay ra \leftrightarrow a=b=c=\sqrt{2} $