已知,x^2-3x+1 = 0 ,可得：x+1/x = 3 ；
因为,(x^4+3x^2+1) / x^2 = x^2+1/x^2+3 = (x+1/x)^2-2+3 = 10 ,
所以,x^2 / (x^4+3x^2+1) = 1/10 .