# Poincare Line Segment - Geometer's Sketchpad

Does anyone know a good method that I can use to build a poincare line segment in The Geometer's Sketchpad? I can build a ray and a line tool no problem but my ray's either disappear or "loop around". Problem 14.2.3 gives you two hints for building a ray tool and I used the second method to build a "reliable" ray by finding inversion points for $B$ and creating an arc through three points. Then finding the intersection of the poincare disk and the arc, hiding the first line I created and then creating another arc through $A$, $B$ and the intersection point on the circle.

I tried using that same method to create a line segment tool, but the line segment falls apart if I try and move any of the points around. I can move the ray around and it is very reliable.

• This question seems to be about a particular software. Perhaps you want to post this at a forum that has support for this software? It might be someone here is able to answer, but perhaps faster help will be found elsewhere. – Pedro Tamaroff Mar 7 '15 at 3:22
• I have but there is not much movement on their message boards. There's a lot of movement here so I was hoping someone would have some helpful hints. – spitfiredd Mar 7 '15 at 12:23
• I don't have Geometer's Sketchpad around (my tool of choice is Cinderella), but I'd use some approach very similar to what you describe. So given $A$ and $B$, invert $B$ in the unit circle to obtain $B'$, then find the circle through $A,B,B'$ and on that circle draw the arc from $A$ to $B$. You don't need to intersect that circle with the unit circle, since you don't need an ideal endpoint. By the way, what document does “Problem 14.2.3” refer to? – MvG Mar 9 '15 at 8:22