=1991-1991*1/2-1991*(1-1/2)*1/3-1991*(1-1/2)*(1-1/3)*1/4-...-1991*(1-1/2)*(1-1/3)*(1-1/4)...1/1991=1991*[1-1/2-(1-1/2)1/3-(1-1/2)(1-1/3)1/4-.-(1-1/2)*(1-1/3)*(1-1/4)...1/1991]分析表达式最后一项可知化...