1 、 3/2 * 4/3* ...20/19* 21/20=21/2 （各项为(n+2)/(n+1)）2、(1-1/2)+(1/2-1/3)+...(1/99-1/100)=1-1/100=99/100
补充的问题 =1-1\2+1\2-1\3+1\3-1\4+1\4-1\5+1\5-1\6+1\6-1\7+1\7-1\8 =1-1\8 =7\8 加分啊
