问题
选择题
已知数列{an}为
|
答案
an=
=1+2+…+n n+1
=n(n+1) 2 n+1
．n 2
∴bn=
=2(1
•n 2 n+2 2
-1 n
)．1 n+2
∴Sn=2[(1-
)+(1 3
-1 2
)+(1 4
-1 3
)+…+(1 5
-1 n-1
)+(1 n+1
-1 n
)]1 n+2
=2(1+
-1 2
-1 n+1
)1 n+2
=
．3n2+5n (n+1)(n+2)
故选A．