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设等差数列{an}的公差为2,前n项和为Sn,则下列结论中正确的是( ) A.S...
设等差数列{an}的公差为2,前n项和为Sn,则下列结论中正确的是( )
A.Sn=nan-3n(n-1)
B.Sn=nan+3n(n-1)
C.Sn=nan-n(n-1)
D.Sn=nan+n(n-1)
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