Definition of a segment
Let A and B be two points on line а. These points form a segment, which is called AB.
Segment AB contains points A and B and all points on line a (marked in red colour the same as line а), which lie between A and B.
The length of the segment АВ =АС+СD+DВ
The length of the segment АD=AC+CD
The length of the segment CB=CD+DB