Đáp án:
`a)`
` => 2x^2 +2y^2 +2z^2 - 2xy - 2yz - 2xz \ge 0`
` => (x^2 - 2xy + y^2) + (y^2 - 2yz + z^2) + (z^2 - 2xz + x^2) \ge 0`
` => (x-y)^2 + (y-z)^2 + (z-x)^2 \ge0`
ĐPCM
`b)`
` => x^2 + y^2 + z^2 - 2xy + 2xz - 2yz \ge 0`
` => ( x - y + z)^2 \ge 0`
ĐPCM
`c)`
` => x^2 + y^2 + z^2 + 3 - 2x - 2y - 2z \ge 0`
` => (x^2 - 2x +1) + (y^2 - 2y +1|) + (z^2- 2z+1) \ge0`
` => (x-1)^2 + (y-1)^2+ (z-1)^2 \ge 0`
ĐPCM