First, the distance between x and y is defined as the absolute value |x − y|, where the absolute value |z| is defined as the maximum of z and −z, thus never negative.
首先，有理数x和y之间的距离定义为绝对值|x − y|，其中绝对值|z|定义为z和−z的最大值，因此总是非负的。