x+x^-1
=（x^1/2+x^-1/2）^2 - 2(x^1/2 * x^-1/2)
=（x^1/2+x^-1/2）^2 - 2
又因为 x+x^-1=3
所以（x^1/2+x^-1/2）^2 =5
x^1/2+x^-1/2=根号5
开始有些急 打错了 不好意思