ta có:2A=(4x2+z2)+(4y2+z2)+(4x2+4y2)mà:$4x^2+z^2\geq4xz;4y^2+z^2\geq4yz;4x^2+4z^2\geq 4xy$$\Rightarrow 2A\geq 4(xy+yz+zx)\geq ............$tự lm tiếp
ta có
:2A=(4x2+z2)+(4y2+z2)+(4x2+4y2)mà:$4x^2+z^2\geq4xz;4y^2+z^2\geq4yz;4x^2+4
y^2\geq 4xy$$\Rightarrow 2A\geq 4(xy+yz+zx)\geq ............$tự lm tiếp