题目内容
已知数列{an}的前n项和Sn=n2-9n,第k项满足5<ak<8,则k等于( )
A.9 | B.8 | C.7 | D.6 |
B
an=
即an=
∵n=1时也适合an=2n-10,∴an=2n-10.
∵5<ak<8,∴5<2k-10<8,
∴
<k<9.又∵k∈N*,∴k=8.

即an=

∵n=1时也适合an=2n-10,∴an=2n-10.
∵5<ak<8,∴5<2k-10<8,
∴


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A.9 | B.8 | C.7 | D.6 |