GeoGebra Construction:


Construct two lines l and m. Mark their point of intersection. Now move a point on one line so that the line rotates while the other line stays fixed. Watch what happens to the point of intersection as the lines pass the position at which they are parallel. Explain how what you observe justifies the choice of just one ideal point on each line even though the line has two ends.


The lines intersect at point C. When I use the move tool to move the points of one line as the other remains finxed, the point of intersection disappears.