an=1+1/2+1/4+……+1/2^(n-1)=(1-1/2^n)/(1-1/2)=2-1/2^(n-1)
所以Sn=2n-[1+1/2+1/4+……+1/2^(n-1)]
=2n-an
=2n-2+1/2^(n-1)
an=1+1/2+1/4+…+1/2^(n-1)=(1-1/2^n)/(1-1/2)=2-1/2^(n-1)
Sn=2-1/2^0+2-1/2^1+2-1/2^2+...+2-1/2^(n-1)
=2n-[1+1/2+1/2^2+...+1/2^(n-1)]
=2n-2+1/2^(n-1)
wakaohaonan
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