应该是|1/21-1/20|﹢|1/21-1/22|+|1/23-1/22|+……+|1/29-1/30|吧,因为|1/30-1/29|没有遵循前面的规律.
|1/21-1/20|﹢|1/21-1/22|+|1/23-1/22|+……+|1/29-1/30|=1/20+（-1/21+1/21）+（-1/22+1/22）+……+（-1/30）=1/60.