`<=>(x^2+4y^2+4z^2-4xy-8yz+4xz)+(x^2-2xy+y^2)+(4z^2-4z+1)=0`

`<=>[(x-2y)^2+2.2z.(x-2y)+4z^2]+(x-y)^2+(2z-1)^2=0`

`<=>(x-2y+2z)^2+(x-y)^2+(2z-1)^2=0`

Dấu "=" xảy ra khi: $\begin{cases} x-2y+2z=0\\x-y=0\\2z-1=0 \end{cases}$

`<=>(x;y;z)=(1;1;1/2)`