Segment Addition Postulate

The segment addition postulate is a fairly obvious-seeming postulate which states that:

If a point B lies on a line segment , then .

That is, the distance from one endpoint to the other is the sum of the distances from the middle point to either endpoint.

Example:

Find the length XY in the figure shown.

By the Segment Addition Postulate, .

Substitute the given values.

Solve for XY.