x,y,z∈R+,xyz=x+y+z,x,y,z\in \mathbb{R+}, xyz=x+y+z, x,y,z∈R+,xyz=x+y+z, 求证xy+xz+yz≥9xy+xz+yz \geq 9xy+xz+yz≥9
最好不要用均值不等式
