因为 (x^2+1)/x=3
所以 (x^2+1)^2/x^2=3^2
(x^4+2x^2+1)/x^2=9
[(x^4+x^2+1)/x^2] +[x^2/x^2]=9
(x^4+x^2+1)/x^2=8
所以 x^2/(x^4+x^2+1)=1/8