Intersecting Line Segments
This Demonstration determines whether or not two line segments intersect. A green dot indicates that the segments intersect and a red dot that they do not. Degeneracy (i.e., when the lines are parallel or coincident) is shown with a red dot in the middle of the screen.
For details, see:
P. Bourke, "Intersection Point of Two Lines (2 Dimensions)." (April 1989)
Wikipedia, "Line-Line Intersection."
"Intersecting Line Segments"
Wolfram Demonstrations Project
Published: September 24 2009