A ________ contains between 15 and 50 retail stores, including a department or variety store, a supermarket, specialty stores, professional offices, and sometimes a bank.
【简答题】已知数列{a n }的首项a 1 =b(b≠0),它的前n项的和S n =a 1 +a 2 +…+a n (n≥1),并且S 1 ,S 2 ,S n ,…是一个等比数列,其公比为p(p≠0且|p|<1), (1)证明:a 2 ,a 3 ,a 3 ,…a n ,…(即{a n }从第二项起)是一个等比数列; (2)设W n =a 1 S 1 +a 2 S 2 +a 3 S 3 +…+a n S n ...