1/1*2+1/2*3+1/3*4.+1/98*99+1/99*100
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+...+(1/99-1/100)
= 1 - 1/100
=0.99
把每一个分数都分成两个分数相减,这样都相消掉了.