Question

设数列{an}的前n项和为Sn=2n²+2n+1 则求通项公式为

Answer 1

Sn=2n²+2n+1

Sn-1=2(n-1)^2+2(n-1)+1

n>=2

an=Sn-Sn-1=4n-2+2=4n

n=1 a1=5

an={5 (n=1)；4n (n>=2)}