极限瘋
幼苗
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S2n=2n+n*(2n-1)d
Sn=n+n(n-1)d/2
4Sn=4n+2(n^2-n)d
S2n/Sn=4 S2n=4Sn
4n+2d(n^2-n)=2n+(2n^2-n)d
整理,得
dn=2n
d=2
S2n=2n+n*(2n-1)d
Sn=n+n(n-1)d/2
4Sn=4n+2(n^2-n)d
S2n/Sn=4 S2n=4Sn
4n+2d(n^2-n)=2n+(2n^2-n)d
整理,得
dn=2n
d=2
an=1+(n-1)*2=2n-1
Sn=n(n+1)-n=n^2
bn=an*2^(n-1)=n^2*2^n/2
1年前
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