Sn=1/2+2/4+3/8+…+n/2^n
1/2Sn= 1/4+2/8+…+(n-1)/2^n+n/2^(n+1)
Sn-1/2Sn=1/2+1/4+1/8+....+1/2^n- n/2^(n+1)
1/2Sn=1-1/2^n-n/2^(n+1)=[2^(n+1)-n-2]/2^(n+1)
Sn=[2^(n+1)-n-2]/2^n
=2-(n+2)/2^n
1/2Sn= 1/4+2/8+…+(n-1)/2^n+n/2^(n+1)
Sn-1/2Sn=1/2+1/4+1/8+....+1/2^n- n/2^(n+1)
1/2Sn=1-1/2^n-n/2^(n+1)=[2^(n+1)-n-2]/2^(n+1)
Sn=[2^(n+1)-n-2]/2^n
=2-(n+2)/2^n
