\(5a^2+ab+5b^2=\frac{1}{2}\left(a+b\right)^2+\frac{9}{2}\left(a^2+b^2\right)\ge\frac{1}{2}\left(a+b\right)^2+\frac{9}{4}\left(a+b\right)^2=\frac{11}{4}\left(a+b\right)^2\)
Do đó:
\(P\ge\frac{\sqrt{11}}{2}\left(x+y\right)+\frac{\sqrt{11}}{2}\left(y+z\right)+\frac{\sqrt{11}}{2}\left(z+x\right)\)
\(P\ge\sqrt{11}\left(x+y+z\right)=\sqrt{33}\)
Dấu "=" xảy ra khi \(x=y=z=...\)