问题
选择题
设Sn为等比数列{an}的前n项和,27a2+a5=0,则
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答案
∵等比数列{an}中,27a2+a5=0,
∴27a1q+a1q4=0,
∵a1≠0,q≠0,
∴q=-3,
∴
=S5 S2
=a1(1-q5) 1-q a1(1-q2) 1-q
=1-q5 1-q2
=-1+243 1-9 61 2
故选:A.
设Sn为等比数列{an}的前n项和,27a2+a5=0,则
|
∵等比数列{an}中,27a2+a5=0,
∴27a1q+a1q4=0,
∵a1≠0,q≠0,
∴q=-3,
∴
=S5 S2
=a1(1-q5) 1-q a1(1-q2) 1-q
=1-q5 1-q2
=-1+243 1-9 61 2
故选:A.