求数列2/2,4/2的平方,6/2的3次方,…,2n/2n次方前n项和Sn 传图片!

2/2,4/2^2,6/2^3,。。。,2n/2^nletS = 1。(1/2)^0+2。(1/2)^1+。。。+n。(1/2)^(n-1) (1)(1/2)S = 1。(1/2)^1+2。(1/2)^2+。。。+n。(1/2)^n (2)(1)-(2)(1/2)S = [1+ 1/2+。。。+1/2^(n-1)] -n。(1/2)^n = 2(1- (1/2)^n) -n。(1/2)^n = 2 - (n+2)(1/2)^nS = 4-2(n+2)(1/2)^nan = 2n/2^n = n(1/2)^(n-1)Sn =b1+b2+。。。+bn =S =4-2(n+2)(1/2)^n