Oriented line - axis
A straight r is oriented when a direction of travel is positive, being indicated by an arrow.
A oriented segment is determined by an ordered pair of points, the first called source of the segment, the second called far end.
A null segment is one whose end coincides with the origin.
If AB is an oriented segment, the oriented segment BA is opposite of AB.
Measurement of a segment
Once a unit of length is set, each oriented segment can be associated with a real, nonnegative number, which is the measure of the segment with respect to that unit. The measure of the oriented segment is its length or yours module. The segment length AB is indicated by .
So the length of the segment AB shown in the figure below is 5 units in length:
= 5 u.c.
Null segments are zero length.