典型的等差数列和等比数列结合题.
Sn=2/2+3/2^2+.+(n+1)/2^n
Sn/2= 2/2^2+.+n/2^n +(n+1)/2^(n+1)
相减得Sn/2=1+[1/2^2+1/2^3+...+1/2^n)]-(n+1)/2^(n+1)=1-[1/2-1/2^n]-(n+1)/2^(n+1)=1/2+(1-n)/2^(n+1)所以Sn=1+（1-n）/2^n
Sn=（1+1）/2+（2+1）/2^2+(3+1)/2^3+``````+(n+1)/2^n=?
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