15. [tex]\sum \frac{1}{\sqrt{2x^2+y^2+3}}=\sum \frac{1}{\sqrt{(x^2+1)+(x^2+y^2)+2}}\leq \sum \frac{1}{\sqrt{2xy+2x+2}}\leq \sqrt{\sum \frac{3}{2xy+2x+2}}=\sqrt{\frac{3}{2}}.\sqrt{\sum \frac{1}{xy+x+1}}[/tex]
Từ giả thiết ta có [tex]xyz\leq 1\Rightarrow \sum \frac{1}{xy+x+1}\leq 1\Rightarrow P\leq \sqrt{\frac{3}{2}}[/tex]