An=（n➕1）/2^n
Sn=2/2+3/2^2+4/2^3+.....+(n+1)/2^n ①
①×1/2:
1/2Sn=2/2^2+3/2^3+4/2^4+....+n/2^n+(n+1)/2^(n+1) ②
①-②:
1/2Sn=1+1/2^2+1/2^3+...+1/2^n-(n+1)/2^(n+1)
=[1-1/2^n]/(1-1/2)-(n+1)/2^(n+1)
=2-4/2^(n+1)-(n+1)/2^(n+1)
=2-(n+5)/2^(n+1)
Sn=4-(n+5)/2^n
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