a平方+3a-1=0,
a≠0,两边同除以a得:
a+3-1/a=0
a-1/a=-3
(a-1/a)²=9
a²+1/a²-2=9
a²+1/a²=11
(a+1/a)²=a²+1/a²+2=11+2=13
a+1/a=±√13
a²-1/a²
=(a+1/a)(a-1/a)
=±3√13