x^2+y^2+z^2-xy-xz-yz=(x-y)^2/2+(y-z)^2/2+(x-z)^2/2>=0 x^2+y^2+z^2>=xy+xz+yz 将已知平方得x^2+y^2+z^2+xy+xa+yz=9/2 x=y=z时成立
