a2-3a-1=0
俩边同除以a得
a-3-1/a=0
a-1/a=3
(a-1/a)^2=a^2-2+1/a^2=9
a^2+1/a^2=11
则（a+1/a)^2=a^2+2+1/a^2=13
则a+1/a=土√13
不懂,祝愉快