求数列2+a,5+a²,8+a³,.(3n-1)+a^n的前几项和

问题描述:

求数列2+a,5+a²,8+a³,.(3n-1)+a^n的前几项和
求详细解答过程谢谢

letS = 1.a^1+2.a^2+.+n.a^n (1)aS = 1.a^2+2.a^3+.+n.a^(n+1) (2)(2)-(1)(a-1)S = n.a^(n+1) - ( a+a^2+...+a^n)= n.a^(n+1) - a( a^n-1)/(a-1)S = [1/(a-1)][n.a^(n+1) - a( a^n-1)/(a-1)]bn= 3n-1cn = a^ndn=bn +...