18/06/17 15:09:46.99 lI+JiKnS.net
>>161
S_n = Σ[k=0，n] (k+1)r^k (r≠1)
とおくと
(1-r)S_n = Σ[k=0,n] (k+1)r^k - Σ[k=0,n] (k+1)r^(k+1)
= Σ[k=0，n] (k+1)r^k - Σ[k=0，n+1] k r^k
= Σ[k=0，n] r^k - (n+1)r^(n+1)
= {1 - r^(n+1)}/(1-r) - (n+1)r^(n+1)
= 1/(1-r) -{(n+1) + 1/(1-r)}r^(n+1),
Σ[k=0，n] (k+1)/3^k = (9/4){1 -(1/3)^(n+1)} - (1/2)(n+1)(1/3)^n
= 9/4 - (2n+5)/{4(3^n)}