问题
解答题
已知各项均正的数列{an}的前n项和为Sn,且2Sn=
(1)求{an}的通项公式 (2)设数列bn=
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答案
(1)∵2Sn=
(an2+an),2Sn+1=1 2
(an+12+an+1)1 2
∴两式相减可得(an+1+an)(an+1-an-1)=0,
∵数列{an}各项均正,
∴an+1-an=1,
∴{an}是以1为公差的等差数列,
∵2S1=
(a12+a1),1 2
∴a1=1
∴an=n;
(2)bn=
(1 2
-1 n
)1 n+2
∴Tn=
(1-1 2
+1 3
-1 2
+…+1 4
-1 n
)=1 n+2
(1+1 2
-1 2
-1 n+1
)=1 n+2
.n(3n+5) 2(n+1)(n+2)