f(x)=x-1/x-3lnx
f'(x)=1+1/x^2-3/x=(x^2-3x+1)/x^2>=0,(x>0)
即有 x^2-3x+1>=0
(x-3/2)^2>=5/4
x-3/2>=根号5/2或x-3/2