lim[n(1-1/2)(1-1/3).(1-1/n-1)] (n趋向于无穷）
=lim(n * 1/2 * 2/3 * 3/4 * ...* (n-2)/(n-1))
=lim(n/(n-1))
=lim(1/(1 - 1/n))
=1