原式=Lim[∑(x^k)-n]／(x-1)(k=1到 n的自然数)x→1=Lim[(x-1)／(x-1)＋(x^2-1)／(x-1)＋...＋(x^n-1)／(x-1)]x→1=Lim{1＋(x+1)＋...＋[x^(n-1)+x^(n-2)+--+x+1)]}x→1=1+2+...+n=n(n+1)／2