Tìm GTNN: `A = x^2 + y^2 + z^2 - yz - 4x - 3y + 2030`

`A=x^2+y^2+z^2-yz-4x-3y+2030`
`=>2A=2x^2+2y^2+2z^2-2yz-8x-6y+4060`
`<=>2A=2x^2-8x+8+y^2-6y+9+y^2-2yz+z^2+z^2+4043`
`<=>2A=2(x-2)^2+(y-3)^2+(y-z)^2+z^2+4043`
Vì `2(x-2)^2+(y-3)^2+(y-z)^2+z^2>=0` với `∀x,y,z`
`=>2A>=4043`
`=>A>=4043/2`
Dấu `=` xảy ra `<=>x=2,y=z=3`
Vậy `A_{min}=....`