>>48
 (1/n)Σ[k=1,n] (n-k-1)r^k
 = (1/n)Σ[k=1,n] (r + r^2 + ・・・・・ + r^k)
 = (1/n){r/(1-r)}Σ[k=1,n] (1-r^k)
 = r/(1-r) -{r/(1-r)}・(r + r^2 + ・・・・・ + r^n)/n
 = r/(1-r) -{r/(1-r)}^2・(1-r^n)/n
 → r/(1-r),    (n→∞)