|1/20-1/19|+|1/19-1/18|+|1/18-1/17|+.+|1/2-1|

问题描述:

|1/20-1/19|+|1/19-1/18|+|1/18-1/17|+.+|1/2-1|

=1/19-1/20+1/18-1/19+1/17-1/18+.1-1/2
=1-1/20
=19/20理由何在?