Question

等差数列{an}中,an=2n+1.若bn=a1+a2+...+an分之n则数列{bn}的前n项和sn

Answer 1

an=2n+1

Sn=a1+a2+...+an=3+5+.+(2n+1)=2(1+2+...+n)+n=n(n+1)+n=n(n+2)

bn=n/Sn=1/(n+2)