幾何

Line

http://mathworld.wolfram.com/Line.html

If an infinite number of points lie next to each other without any space between them, a line is formed. In other words, a line consists of an infinite number of points. Graphically, a line is represented by a straight segment with arrows at the ends (Figure 1.3). A line is denoted by the labels of its end points with “  “ above the letters. For example, the line in Figure 1.3 is represented by  A line always extends indefinitely from the both ends. It has only one dimension, length. It can also be viewed as the path of a point that has been and will always be traveling without changing direction.

When three or more points all lie on the same line, they are called collinear. Two points will always lie on the same line

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