Sn=1/2^1+1/2^2+1/2^3+……1/2^n
(1/2)Sn=1/2^2+1/2^3+……1/2^n+1/2^(n+1)
所以Sn-(1/2)Sn=1/2^1-1/2^(n+1)
所以Sn=1-1/2^n