x2+y2+z2=9x^2+y^2+z^2=9x2+y2+z2=9
1<=x<=21<=x<=21<=x<=2
1<=y<=21<=y<=21<=y<=2
1<=z<=21<=z<=21<=z<=2
求证:x+y+z>=5x+y+z>=5x+y+z>=5
各位能不能帮忙想想思路 打扰了