`b)`

`(x - y + z)^2 >= 0`

`=> [(x-y) + z]^2 >= 0`

`=> (x-y)^2 + 2(x-y).z + z^2 >= 0`

`=> x^2 - 2xy + y^2 + 2xz - 2yz + z^2 >= 0`

`=> x^2 + y^2 + z^2 - 2xy - 2yz + 2zx >= 0`

`=> x^2 + y^2 + z^2 >= 2xy + 2yz - 2zx (đpcm)`

Vậy `x^2 + y^2 + z^2 >= 2xy + 2yz - 2zx`