1=4
b2=8
d=4
bn=4n
nsn=3n-2
(n+1)s(n+1)=3n+1
a(n+1)=(3n+1)/(n+1)-(3n-2)/n
所以an=(3n-2)/n-(3n-5)/(n-1) (n>=2)
n=1 a1=1