1.2 Line Segment
A line segment is a part of a line. It has a fixed length and consequently two end points. They are used to name the line segment (figure 1.8).
Seg. PQ is a segment of line AB. A line contains infinite segments and if two segments on a line have a common end point, they can be added ( figure 1.9).
Seg. PQ and Seg.QR are two segments on line l and they have a common end point Q. Therefore Seg.PQ + Seg.QR = Seg.PR.
On segment Seg.PR there exists a point M. Therefore seg. PM + seg.MR = seg.PR or seg. PM = seg. MR , it implies that seg.PR = 2 seg. PM = 2 seg. MR. In other words it means that M is equidistant from P & R. Therefore, M is the midpoint of seg.PR. Every segment has one and only one midpoint.