$=( 9x^2 +36xy +36y^2 ) +(18y^2 -24yz + 8z^2)+ (8x^2 +16xz + 8z^2) + 2x^2 +5 \geqslant 0$$\Leftrightarrow 9(x+2y)^2 + 2(3y - 2z)^2 + 8(x-z)^2 + 2x^2 + 5\geqslant 0$$\Rightarrow :đpcm$
$=( 9x^2 +36xy +36y^2 ) +(18y^2 -24yz + 8z^2)+ (8x^2 +16xz + 8z^2) + 2x^2 +5 \geqslant 0$$\Leftrightarrow 9(x+2y)^2 + 2(3y - 2z)^2 + 8(x-z)^2 + 2x^2 + 5\geqslant 0$$\Rightarrow :đpcm$đúng thì tick v bên trái nha bạn
$=( 9x^2 +36xy +36y^2 ) +(18y^2 -24yz + 8z^2)+ (8x^2 +16xz + 8z^2) + 2x^2 +5 \geqslant 0$$\Leftrightarrow 9(x+2y)^2 + 2(3y - 2z)^2 + 8(x-z)^2 + 2x^2 + 5\geqslant 0$$\Rightarrow :đpcm$