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