已知等差数列{an}满足:a3=7,a5+a7=26.{an}的前n项和为Sn.(1)求an及Sn; (2)令bn=1/

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已知等差数列{an}满足:a3=7,a5+a7=26.{an}的前n项和为Sn.(1)求an及Sn; (2)令bn=1/an^2-1(n属于N),求数...
已知等差数列{an}满足:a3=7,a5+a7=26.{an}的前n项和为Sn.
(1)求an及Sn;
(2)令bn=1/an^2-1(n属于N),求数列{bn}的前n项和Tn.

1)a5+a7=2a6=26得a6=13
d=(a6-a3)/(6-3)=(13-7)/3=2
a1=a3-2d=3
Sn=3n+(1/2)n(n-1)·2=n^2+2n
2)an=2n+1,然后写出bn的式子,利用拆项求和,得Tn=n/[4(n+1)]

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