The segment addition postulate states that if a line segment has three points, then this line segment may be considered two line segments. An example is a line featuring points A, B and C with A and C being the endpoints. In this example, AC = AB + BC.
To determine the length of AB, one must subtract BC from AC. If the length of AC is 18 and the length of BC is 4, then using this formula yields 18 - 4 = 14, so the length of AB is 14.
This postulate also allows a line segment that has only two known points to be broken into two line segments with the addition of a third point in between the endpoints. This is useful for proofs in geometry and analysis.