an=2^n+n,求Sn
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an=2^n+n,求Snan=2^n+n,求Snan=2^n+n,求Snan=2^n+nSn=2+2^2+2^3+……+2^n+1+2+3+……+n=2(1-2^n)/(1-2)+(1+n)n/2=2
an=2^n+n,求Sn
an=2^n+n,求Sn
an=2^n+n,求Sn
an=2^n+n
Sn=2+2^2+2^3+……+2^n+1+2+3+……+n
=2(1-2^n)/(1-2) + (1+n)n/2
=2(2^n-1) +(1+n^2)/2
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