(12)
an = n.(1/2)^n
Sn = a1+a2+....+an
= 1.(1/2)^1+2.(1/2)^2+.....+n.(1/2)^n (1)
(1/2)Sn = 1.(1/2)^2+2.(1/2)^3+.....+n.(1/2)^(n+1) (2)
(1)-(2)
(1/2)Sn = [1/2+1/2^2+...+1/2^n) - n.(1/2)^(n+1)
= [ 1 -(1/2)^n ] -n.(1/2)^(n+1)
Sn = 2[ 1 -(1/2)^n ] -n.(1/2)^n
= 2 - (n+2).(1/2)^n
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