由均值不等式：
分母=x^2+y^2+z^2=(x^2+y^2/2)+(y^2/2+z^2/2)>=√2*xy+√2*yz=√2(xy+yz)
所以(xy+yz)/(x^2+y^2+z^2)<=(xy+yz)/[√2(xy+yz)]=1/√2
所以(xy+yz)/(x^2+y^2+z^2)有最大值1/√2