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## Basic geometry and measurement

### Course: Basic geometry and measurement>Unit 3

Lesson 1: Parts of plane figures

# Lines, line segments, & rays

Learn the difference between lines, line segments, and rays.  Created by Sal Khan.

## Want to join the conversation?

• would an infinite line and an infinite ray be equally long? That's my question.
• no, look at set theory as an example. if there is a set that extends infinitely to all the positive numbers, and then there is a set that extends infinitely in both directions, with negative numbers and positive numbers, they are not equal set, because even though both are infinite, you cannot match up each element os the positive set with each element of the negative set. In other words, for every centimeter of the ray, there would be twice as many centimeter of line, therefore the line is longer
• Is line EF and line FE the same?
• Yup!
When naming a line using just two points, order usually does not matter.
• Does anyone else remember a ray by think of a ray of sunshine, it starts at the sun can't get in so it goes out?
• yes. It's how I originally thought of it when I heard of it
• Are the lines of longitude and latitude really mathematical lines? They do not go on forever and neither are they line segments since they do not have a starting point or ending point...
• The Earth is considered an oblique spheroid (in other words an irregular sphere). So, the longitude and latitude lines aren't really circles as they are ellipses.
• What is the best way to get better at geometry or any other type of math?
• Practice. The more you work at answering these types of problems, the more your brain will become accustomed to them. You'll get faster and more accurate at solving math problems.
• How come lines have no thickness? Isn't it as thick as the line?
• When you draw a line it has thickness, but that is just a representation. The abstract idea of a line, however, does not have any thickness.
• Would two lines that are coincident (identical lines) have infinite intersection? I know that two distinct lines intersect at one or no points. But two coincident lines?
• Okay so lines can extend in two directions but outwards,
what if we want them to extend inwards and collapse at a point?