I saw this question in the review queue for closing:
It's phrased as a pure math question, but the applications for gamedev are obvious. The question can easily be rephrased as something like, "How do I detect how close a game entity is to the line between two waypoints?" Suddenly, on-topic! Assuming there are no questions already about this (there don't seem to be in this case).
More explicitly asked: Should we actually edit questions such as these so that they reflect the purpose of the site better, or is it better to let these get closed and wait for somebody to post the question in game terms?