已知数列a n 是首项为1的等比数列,S n 是a n 的前n项和,且S 6 =9S 3 ,则数列a n 的通项公式是( )
A.
2 n-1
B.
2 1-n
C.
3 1-n
D.
3 n-1
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举一反三
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