考慮 f(t)=sqrt(t) (x >= 0). f"(t)<=0, 是凹函數。由JESEN不等式：
LHS <= 3 sqrt((x+yz + y+zx + z+xy)/3) = sqrt(3(1 + xy+yz+zx))
因 xy+yz+zx <= (x+y+z)^2/3 = 1/3,代入上式右邊即得。
LHS <= 3 sqrt((x+yz + y+zx + z+xy)/3) = sqrt(3(1 + xy+yz+zx))
因 xy+yz+zx <= (x+y+z)^2/3 = 1/3,代入上式右邊即得。
