若數(shù)列｛An}的前n項(xiàng)和Ｓn=2An+1,問數(shù)列｛Ａn｝是否成等比數(shù)列？說明理由，并求出An

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Ｓn=2An+1 == Ｓn-1 =2A(n-1)+1 == Ｓn - Ｓn-1 = An = (2An+1)-[2A(n-1)+1] = 2An -2A(n-1)An/A(n-1) = 2== ｛Ａn｝成等比數(shù)列Ｓ1 = A1 = 2A1 +1 == A1 = -1== An = 1 - 2^n