已知数列{an}的前n项和为Sn,且Sn=n(n+1)(n∈N*),(Ⅰ)求数列{an}的通项公式an(Ⅱ)数列{bn}的通

2025-04-07 14:38:16
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回答1:

(Ⅰ)∵数列{an}的前n项和为Sn
且Sn=n(n+1)(n∈N*),
∴a1=S1=1×(1+1)=2,
an=Sn-Sn-1=n(n+1)-(n-1)n
=(n2+n)-(n2-n)
=2n.
(Ⅱ)∵an=2n,∴bn=

1
an?an+1
=
1
2n?2(n+1)
=
1
4
(
1
n
?
1
n+1
)

∴Tn=
1
4
1?
1
2
+
1
2
?
1
3
+…+
1
n
?
1
n+1

=
1
4
(1?
1
n+1
)

=
n
4n+4