2(x+1/x)²-5(x+1/x)+2=0
设x+1/x=t
则：2t²-5t+2=0
(2t-1)(t-2)=0
t=1/2或t=2
(1) x+1/x=1/2
两边同乘以2x得：
2x²-x+2=0
因为△=1-16=-15