采用Sn-q倍Sn,错位相减法!
an=(2n-1)*(1/2)^n
Sn=1*(1/2)+3*(1/2)^2+5*(1/2)^3+……+(2n-1)*(1/2)^n
0.5Sn=1*(1/2)^2+3*(1/2)^3+……+(2n-3)*(1/2)^n+(2n-1)*(1/2)^(n+1)
两式相减：
0.5Sn=1*(1/2)+2*(1/2)^2+2*(1/2)^3+……+2*(1/2)^n-(2n-1)*(1/2)^(n+1)
Sn=1+4*[(1/2)^2+(1/2)^3+……+(1/2)^n]-2(2n-1)*(1/2)^(n+1)
=3-4*(1/2)^n+(2n-1)*(1/2)^n
=(2n-5)*(1/2)^n+3