>213,214,217,220,224,228
　Ｓ_j = Σ[i=0,∞) X_i = (p^j)Σ[i=0,∞) Ｃ[i+j-1,i] q^i とおく。
　Ｃ[i+j-1,i] = Ｃ[i+j,i] - Ｃ[(i-1)+j,i-1] (i≧1), Ｃ[j-1,0] = 1 = Ｃ[j,0]
　∴ Ｓ_j/(p^j) = (1-q)･Ｓ_{j+1}/[p^(j+1)],
　∴ Ｓ_j = {(1-q)/p}Ｓ_{j+1}, Ｓ_1 = pΣ[i=0,∞) q^i = p/(1-q).
　∴ Ｓ_j = Ｓ_1･{p/(1-q)}^(j-1) = {p/(1-q)}^j.
ぬるぽ