Actually for a triangle in lisp its easy just pick the 3 points that make the triangle this gives ang1 dist1 ang2 dist2 and dist3 then just pick a point for the start of your new line pick a brg angle and your line will appear.
(setq pt1 (getpoint "\nPick 1st pt"))(setq pt2 (getpoint "\nPick 2nd pt"))(setq pt3 (getpoint "\nPick 3rd pt")); you can get angels and distances(setq ang1 (angle pt1 pt2))(setq dist3 (distance pt1 pt3))(setq newlen (+ dist3 100.0))(setq pt4 (getpoint "\npick line start pt"))(setq pt5 (getpoint pt4 "\npick line end pt"))(setq ang4 (angle pt4 pt5))(setq pt6 (polar pt4 ang4 newlen))(command "line" pt4 pt6 ""); all done this took about 5 minutes I spent about an hour yesterday trying to figure it out wit parametrics.
