1/1x2＋1/2x3＋1/3x4＋1/4x5＋1/5x6＋1/6x7＋1/7x8＋1/8x9＝?

题目

1/1x2＋1/2x3＋1/3x4＋1/4x5＋1/5x6＋1/6x7＋1/7x8＋1/8x9＝?
急!

答案

=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+1/8-1/9
=1-1/9
=8/9