an=2n-1+(1/2)n次方
a(1)=2*1-1+(1/2)^1
a(2)=2*2-1+(1/2)^2
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a(n)=2*n-1+(1/2)^n
s(n)=2*(1+2+...+n)-n+[1/2+(1/2)^2+.+(1/2)^n]
=n(n+1)-n+1-1/2^n
=n^2+1-1/2^n