N points on the 1. plane that are not on a straight line can draw at most n(n- 1)/2 straight lines, n(n- 1) rays and n(n- 1)/2 line segments.

2. There are n points on a straight line, which can form n(n- 1)/2 line segments and n(n- 1) rays.

3.A/B/C/D is four points in turn on the straight line 1, while AC = 5 and BD = 4, then the difference between AB and CD is 1.