已知数列{a n },构造一个新数列a 1 ,(a 2 -a 1 ),(a 3 -a 2 ),…,(a n -a n-1 ),…,此数列是首1,公比为 1 3 的等比数列. (1)求数列{a n }的通项; (2)求数列{a n }的前n项和S n .
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