求数列{2n·3的n次方}的前n项和_数学解答

求数列{2n·3的n次方}的前n项和

人气：218 ℃　时间：2020-06-11 09:28:04

解答

letS = 1.3^1+2.3^2+...+n.3^n(1)3S =1.3^2+2.3^3+...+n.3^(n+1)(2)(2)-(1)2S = n.3^(n+1) -(3+3^2+...+3^n) =n.3^(n+1) -(3/2)(3^n-1) = 3/2...