(x-1/x)^2=1 => x^2+1/x^2=3
(x^2+1/x^2)^2=9 =>x^4+1/x^4=7
(x^4+1/x^4)^2=49=> （x^4-1/x^4)^2=45
因为x>0,且x-1/x>0,故,x^4-1/x^4>0,所以,x^4-1/x^4=3根号5