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已知数列an是首项为1的等比数列,Sn是an的前n项和,且S6=9S3,则数列a...
已知数列an是首项为1的等比数列,Sn是an的前n项和,且S6=9S3,则数列an的通项公式是( )
A.2n-1
B.21-n
C.31-n
D.3n-1
考点分析:
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