已知数列{an}的前n项和为Sn=-n2-2n,求an

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已知数列{an}的前n项和为Sn=-n2-2n,求an

已知数列{an}的前n项和为Sn=-n2-2n,求an
已知数列{an}的前n项和为Sn=-n2-2n,求an

已知数列{an}的前n项和为Sn=-n2-2n,求an
an=Sn-Sn-1=-1-2n

Sn=-n²-2n
an=Sn-S(n-1)
=-n²-2n-[-(n-1)²-2(n-1)]
=-n²-2n+n²-2n+1+2n-2
=-2n-1

Sn-Sn-1=an,再单独验证a1详细自己算