title: 一个难题设x,y,z为正实数.试证1/(x^2+2yz)+1/( 
desc: 设x,y,z为正实数.试证
  1/(x^2+2yz)+1/(y^2+2zx)+1/(z^2+2xy)>=
  2/(yz+zx+xy)+1/(x^2+y^2+z^2)
answer: 设x,y,z为正实数.试证 
  1/(x^2+2yz)+1/(y^2+2zx)+1/(z^2+2xy)>= 
  2/(yz+zx+xy)+1/(x^2+y^2+z^2)
  
  证明如下:
  