2/2,4/2^2,6/2^3,...,2n/2^nletS = 1.(1/2)^0+2.(1/2)^1+...+n.(1/2)^(n-1) (1)(1/2)S = 1.(1/2)^1+2.(1/2)^2+...+n.(1/2)^n(2)(1)-(2)(1/2)S...