当x=1时,Sn=1+3+5+……+（2n-1）=2n*n/2=n^2
当x≠1时,Sn=1+3x+5x^2+……+(2n-1)x^(n-1)
xSn= x+3x^2+……+(2n-3)x^(n-1)+(2n-1)x^n
(1-x)Sn=1+2x+2x^2+……+2x^(n-1)-(2n-1)x^n
=2x[1-x^(n-1)]/(1-x)+1-(2n-1)x^n
Sn=2x[1-x^(n-1)]/(1-x)^2+[1-(2n-1)x^n]/(1-x) x≠1