x²-5x-1=0
除以x得 x-5-1/x=0
x-1/x=5
(1)x²+（1/x²)
=(x-1/x)²+2
=5²+2
=27
(x+1/x)²=(x-1/x)²+4
=5²+4
=29
∴x+1/x==±√29