1.数列{an}的通项公式为an=,若{an}的前n项和为24,则n=(  )
A.25                                   B.576 
C.624                                  D.625
答案 C
解析 an=-,所以Sn=(-)+(-)+…+(-)=-1,令Sn=24得n=624.故选C.
2.数列{(-1)n(2n-1)}的前2020项和S2020等于(  )
A.-2018                               B.2020 
C.-2017                               D.2017
答案 B
解析 S2020=-1+3-5+7+…-(2×2019-1)+(2×2020-1)==2020.故选B.
3.已知数列{an}中的前n项和Sn=n(n-9),第k项满足7<ak<10,则k等于(  )
A.7                                    B.8 
C.9                                    D.10