错位相减
Sn =1*3+3*3^2+5*3^3+.+(2n-3)*3^(n-1)+(2n-1)*3^n (1)
同乘以3
3Sn = 1*3^2+3*3^3+.+(2n-3)*3^n+(2n-1)*3^(n+1) (2)
(1)-(2)
-2Sn =3+2[3^2+3^3+.+3^n]-(2n-1)*3^(n+1)
-2Sn=3+2*[9-3^(n+1)]/(1-3)-(2n-1)*3^(n+1)
-2Sn=3+3^(n+1)-9-(2n-1)*3^(n+1)
Sn=3+(n-1)*3^(n+1)