应该是a^2-3a+1=0
a^2-3a+1=0
等式两边同除以a
a-3+1/a=0
a+1/a=3
a^2+1/a^2=(a+1/a)^2-2=3^2-2=7
(a-1/a)^2=(a+1/a)^2-4=3^2-4=5