Definition:[br]The [b]projection[/b] of a point onto a line is the foot of the perpendicular segment drawn from the point to the line.[br][br]In the diagram, [math]A'[/math] is the projection of [math]A[/math] onto line [math]l[/math] and [math]B'[/math] is the projection of [math]B[/math] onto line [math]l[/math].[br][br]Similarly, we can talk about the projection of an entire segment onto a line, which is just the segment joining the projections of the endpoints of the original segment. In the diagram, [math]\overline{A'B'}[/math] is the projection of [math]\overline{AB}[/math] onto line [math]l[/math].[br][br]Drag [math]A[/math] and [math]B[/math] around to see how their projections move along line [math]l[/math].