|1/2-1|+|1/2-1/3|+|1/3-1/4|+|1/4-1/5|…+|1/9-1/10|
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5…+1/9-1/10
=1-1/10
=9/10