裂项相消求和：数列{an}中,an=1/(n+1)+2/(n+1)+3/(n+1)+……+n/(n+1),bn=2/(an*a（n_数学解答

裂项相消求和：数列{an}中,an=1/(n+1)+2/(n+1)+3/(n+1)+……+n/(n+1),bn=2/(an*a（n+1）)
,求数列{bn}的前n项

人气：241 ℃　时间：2020-09-26 10:39:30

解答

an=1/(n+1)+2/(n+1)+3/(n+1)+……+n/(n+1)=(1+2+3+.+(n+1))/(n+1)
=((n+2)(n+1)/2)/(n+1)=(n+2)/2
所以a(n+1)=(n+3)/2
所以bn=2/(an*a(n+1))=2/((n+2)(n+3)/4)=8/(n+2)(n+3)=8(1/(n+2)-1/(n+3))
所以Sn=8(1/3-1/4+1/4-1/5+.+1/(n+2)-1/(n+3))=8(1/3-1/(n+3))=8n/3(n+3)