等差数列{an}中a2=8,S6=66.设bn=2/[(n+1)an],Tn=b1+b2+…+bn,

来源:学生作业帮助网 编辑:六六作业网 时间:2024/11/23 07:04:41
等差数列{an}中a2=8,S6=66.设bn=2/[(n+1)an],Tn=b1+b2+…+bn,等差数列{an}中a2=8,S6=66.设bn=2/[(n+1)an],Tn=b1+b2+…+bn,

等差数列{an}中a2=8,S6=66.设bn=2/[(n+1)an],Tn=b1+b2+…+bn,
等差数列{an}中a2=8,S6=66.设bn=2/[(n+1)an],Tn=b1+b2+…+bn,

等差数列{an}中a2=8,S6=66.设bn=2/[(n+1)an],Tn=b1+b2+…+bn,
S6=3(a2+a5) 所以a5=14
d=(a5-a2)/3=2
an=a2+(n-2)d=2n+4=2(n+2)
bn=1/[(n+1)(n+2)]=1/(n+1) - 1/(n+2)
Tn=1/2-1/3+1/3-1/4+1/4-1/5+……+1/(n+1) - 1/(n+2)
=1/2 - 1/(n+2)