Ta có: \(\left(x-y\right)^2\ge0\Leftrightarrow x^2+y^2\ge2xy\)
\(\left(y-z\right)\ge0\Leftrightarrow y^2+z^2\ge2yz\)
\(\left(z-x\right)^2\ge0\Leftrightarrow z^2+x^2\ge2zx\)
\(\left(x-1\right)^2\ge0\Leftrightarrow x^2+1\ge2x\)
\(\left(y-1\right)^2\ge0\Leftrightarrow y^2+1\ge2y\)
\(\left(z-1\right)^2\ge0\Leftrightarrow z^2+1\ge2z\)
Cộng lại vế với vế ta được:
\(3\left(x^2+y^2+z^2\right)+3\ge2xy+2yz+2zx+2x+2y+2z\)
\(\Leftrightarrow Q\ge\frac{2\left(x+y+yz+xy+yz+zx\right)-3}{3}=3\)
Dấu \(=\)khi \(x=y=z=1\).
Ta có: \(x+y+z+xy+yz+xz\le x+y+z+\frac{\left(x+y+z\right)^2}{3}\)
=> \(\left(x+y+z\right)^2+3\left(x+y+z\right)\ge3.6=18\)
<=> \(\left(x+y+z\right)^2+3\left(x+y+z\right)-18\ge0\)
<=> \(\left(x+y+z-3\right)\left(x+y+z+6\right)\ge0\)
<=> \(x+y+z\ge3\)(vì x + y + z + 6 > 0 vì x,y,z > 0)
Do đó: \(Q=x^2+y^2+z^2\ge\frac{\left(x+y+z\right)^2}{3}=\frac{3^2}{3}=3\)
Dấu "=" xảy ra<=> x = y= z và x + y + z = 3 <=> x = y = z = 1
Vậy MinQ = 3 <=> x = y= z = 1
