\(x^2+y^2+z^2+x^2y^2+y^2z^2+z^2x^2=6\)
\(\Leftrightarrow x^2+y^2+z^2=6-x^2y^2-y^2z^2-z^2x^2\)
Ta có:
\(P^2=x^2+y^2+z^2+2xy+2yz+2zx\)
\(P^2=6-x^2y^2-y^2z^2-z^2x^2+2xy+2yz+2zx\)
\(P^2=9-\left(xy-1\right)^2-\left(yz-1\right)^2-\left(zx-1\right)^2\le9\)
\(\Rightarrow P\le3\)
\(P_{max}=3\) khi \(x=y=z=1\)