a²-3a+1=0
等式两边同除以a
a-3+1/a=0
a+1/a=3
a²+1/a²
=(a+1/a)²-2
=3²-2
=9-2
=7
(a-1/a)²
=(a+1/a)²-4
=3²-4
=9-4
=5