a-1/a=2(平方）
(a-1/a)^2=4
a^2-2+1/a^2=4
a^2+1/a^2=6
a^2+2+1/a^2=6+2
(a+1/a)^2=8
a+1/a=±2√2
(a^4+1)/a^2
=a^2+1/a^2
=6