已知数列{a
n}满足:a
1=1,

=2(n十1)a
n+n(n+1),(

),
(I)若

,试证明数列{b
n}为等比数列;
(II)求数列{a
n}的通项公式a
n与前n项和Sn.
解:(Ⅰ)

, ………2分

,即

,

,

是以2为首项,2为公比的等比数列. ………5分
(Ⅱ)由(Ⅰ)知

………7分
∴



. ………9分
令

,
则

,
两式相减得:

,

. ………11分
∴

. ………12分
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