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an=1+2+……+2^(n-1)=1*(1-2^n)/(1-2)=2^n-1 所以Sn=2^1-1+2^2-1+……+2^n-1 =(2+……+2^n)-(1+1+……+1) =2*(1-2^n)/(1-2)-n =2^(n+1)+2-n 滿意望採納!
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