Question

数列{an}前n项和Sn=n²+1,写出数列的前5项

Answer 1

a1=s1=1^2+1=2

sn=n^2+1

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

sn-s(n-1)=2n-1

an=2n-1(n>=2)

a2=2*2-1=3

a3=2*3-1=5

a4=2*4-1=7

a5=2*5-1=9