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
2
(an2+an
(1)求{an}的通项公式
(2)设数列bn=
1
anan+2
,求数列{bn}的前n项的和Tn
答案

(1)∵2Sn=

1
2
(an2+an),2Sn+1=
1
2
(an+12+an+1

∴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=

1
2
(a12+a1),

∴a1=1

∴an=n;

(2)bn=

1
2
1
n
-
1
n+2

∴Tn=

1
2
(1-
1
3
+
1
2
-
1
4
+…+
1
n
-
1
n+2
)=
1
2
1+
1
2
-
1
n+1
-
1
n+2
)=
n(3n+5)
2(n+1)(n+2)

单项选择题
单项选择题