数学问题若x+1/x等于2根号5;则X
若x+1/x等于2根号5;则X-1/x等于多少
(x+1/x)^2=x^2+1/x^2+2=20,得x^2+1/x^2=18 (x-1/x)^2=x^2+1/x^2-2=16 x-1/x=4或-4