设Sn=C1+C2+...+Cn=1x3^0+3x3^1+5x3^2+...+(2n-1)x3^(n-1)
3Sn=1x3^1+3x3^2+5x3^3+...+(2n-1)x3^n
3Sn-Sn=2Sn=(2n-1)x3^n - 2(3^1+3^2+...+3^(n-1)) - 1x3^0
又,3^1+3^2+...+3^(n-1)=(3^n - 3^1)/2
则2Sn=(2n-1)x3^n - (3^n-3^1) - 1=2(n-1)x3^n +2
Sn=(n-1)x3^n + 1