Dùng BĐT Cauchy-Schawrz ta có: + 9=$(\sqrt{2-x}+\sqrt{2-y}+\sqrt{2-z})^{2}\leqslant3(6-x-y-z) $ +81=$(\sqrt{8+x}+\sqrt{8+y}+\sqrt{8+z})^{2}\leqslant3(2 4+x+y+z) $ Cộng hai vế ta có: $90\leqslant3(6-x-y-z+24+x+y+z)=90 $ Dấu bằng xảy ra khi x=y=z=1 [RIGHT][I][B]Nguồn: MathScope.ORG[/B][/I][/RIGHT] |