n>=2时S(n-1)=(n-1)²+2(n-1)=n²-1所以此时an=Sn-S(n-1)=2n+1a1=S1=1+2=3,满足an=2n+1所以an=2n+1所以bn=log5(2n+1)
Sn 和an啥关系？
