|1/21-1/20|+|1/22-1/21|+|1/23-1/22|+...+|1/35-1/34|
(分母大的反而小,所以去掉绝对值时前面要加负号)
=-1/21+1/20-1/22+1/21-1/23+1/22-...-1/35+1/34
=1/20-1/35
=3/140