[x^(2/3)+x^(-2/3)]^2=x^3+x^(-3)+2
[x+x^(-1)]^3=x^3+3x+3x^(-1)+x^(-3)=x^3+x^(-3)+9=3^3=27
x^3+x^(-3)=27-9=18
x^(2/3)+x^(-2/3)=根号（18+2）=2*根号5
^表示乘方