x^1/2+x^(-1/2)=3
[x^1/2]^2+2x^(1/2)x^(-1/2)+[x^(-1/2)]^2=9
x+2+1/x=9
x+1/x-7=0
x=1/4 (3 - 根号[5])^2
x= 1/4 (3 + 根号5])^2