设前n项和为T
则T=1/2+3/4+5/8+7/16+.+2n-1/2^n
T/2= 1/4+3/8+5/16+.+2n-3/2^n+(2n-1)/2^(n+1)
上面减下面得：T/2=1/2+1/2+1/4+1/8+...+1/2^(n-1)-(2n-1)/2^(n+1)
所以T=3-(2n+3)/2^n