a/a方+a+1=1/5
1/(a+1+1/a)=1/5
a+1/a=5-1=4
a方/a的4次方+5a方+1
=1/(a^2+5+1/a^2)
=1/[(a+1/a)^2+3]
=1/(16+3)
=1/19