=(1-1/2）+(1/2-1/3）+（1/3-1/4）+（1/4-1/5)+(1/5-1/6）+（1/6-1/7)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-1/7
=6/7