已知数列an的前n项和为sn=n²+n,求它的通项公式an
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已知数列an的前n项和为sn=n²+n,求它的通项公式an
已知数列an的前n项和为sn=n²+n,求它的通项公式an
已知数列an的前n项和为sn=n²+n,求它的通项公式an
an=Sn-Sn-1=n²+n-(n-1)²-(n-1)
=n²+n-n²-1+2n-n+1
=2n
a1=s1=2
an=Sn-s(n-1)=n²+n-(n-1)²-(n-1)=2n