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Lesson 1: Types of plane figures

Terms & labels in geometry

Explore geometry fundamentals, including points, line segments, rays, and lines. Understand dimensions and how these elements form shapes and patterns. Learn key geometric terms like colinear points, midpoints, and vertices, and enhance your knowledge of geometry. Improve your skills and discover the world of shapes and space. Created by Sal Khan.

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• This video has a number of terms defined to help understand geometry. Where can I find each term discussed in this video?
• The following terms can be found at these approximate time markers:
Definition of name Geometry –
Point (0 dimensions) -
Line segment -
End Points –
Ray -
Line -
Collinear -
Midpoint –
Plane (2 dimensions) -
3 dimensions –
• Earth Measurement. can someone explain?
• He's just referring to the origin of the term; only the Greeks know precisely why, but it seems likely that it refers to the measuring of "earth", meaning "the land" or "the physical world" in its various aspects rather than an actual attempt to measure our planet.
• At , Sal mentions that some things have more than three dimensions. Are there stuff on earth, in the solar system, or even in the world that have more than three deminsions, or are they just theoretical, like a point or a line etc.? If there are stuff, can we see them?
• Mostly we have to use our imaginations to think about things that have more than three dimensions. Sometimes theoretical scientists like to think of time being the fourth dimension, so if you think about an balloon being inflated over time, that's maybe a little bit like a four dimensional "hypercone" that is a sphere at every instant just like a normal cone is a circle anywhere you make a flat slice across it.
• Do scalars just have magnitude and vectors have magnitude and direction?
• Yes to be 2 dimensional you must be able to go forward and backward
• So , the line has no end?
• Yes, a line goes on forever, but a line segment is only part of a line so it stops in both ways.
• Can someone summarize what terms were taught in this video and what each term means?
• Here are all the terms taught in the video, and the definitions for each (credit to Ed for the timestamps):

Definition of name Geometry

The definition of the name geometry is "Earth measurement", which basically defines all of what Geometry is about. Geometry is the study of understanding how shapes and space and things that we see relate to each other. This is why you learn about many different shapes such as Triangles, Squares, and much more.

Point (0 dimensions) -

Sal explains that a point is 0 dimensions and how you cannot really move one point around and still call it the same point. In addition, in order to recognize a point, you would need to give it a name. In Geometry, points are often labeled to as letters in the English alphabet.

Line segment
-

Now what if you wanted to get from one point to another? Sal explains that if you wanted to do that, you would need to create a straight line in between them. A straight line connecting two points is referred to as a line segment. These are most used in Geometry because they have a finite length.

End Points

What if you wanted to label your line segment? You would do so by looking at its endpoints, which are the literal endpoints of a line segment. For example, if AB were the endpoints of your line segment, then the line segment would be known as AB (with a straight line on top of AB to show that it's a line segment). The order doesn't matter, so AB could also be referred to as BA (with a straight line still on top of it). Line segments, unlike points, are one-dimensional, because they can travel left and right from one point to another. However, they have no width, so that reason doesn't let it go any further then one-dimension. If you wanted to specify the length, you would measure it and write it like this. For example: AB = 5 (remember the straight line on top of AB!)

Ray -

What if you wanted to keep going in one direction, and not have a finite length? You would call that a ray. For example: The line would start at A, and it would continue going straight, regardless of where the other point D is located. The starting point of a ray would be called a vertex, and in this case, would be the point A. To specify a ray, you would write the two points, which are AD in this case, and put an arrow on top of it to show that it continues in one direction. Unlike in a line segment, the order matters in which you put the points in. You cannot write DA with an arrow on top, because it looks as if the ray's vertex is D, and goes past A instead.

Line -

What if you wanted to keep going in both directions? That would be called a line. For example: You have two points; E and F. The line in between them would continue travelling outside of these points, regardless of where they are. To specify a line, you would write both the points down, which in this case is E and F, and then draw a double-sided arrow on top of it. This shows that the line does not stop and keeps going in both directions.

Collinear -

What happens if you have more then two points on any line? For example: You have three points; XYZ on a line. These points would be called collinear, since there are three points on it.

Midpoint

If there was a point directly in the middle of the other two endpoints in a line segment, we would refer to that as a midpoint. This is because this point is in the "middle" of the line segment.

Plane (2 dimensions) -

Sal now explains the concept of something having two dimensions. Something has two dimensions if it can go backwards or forwards in two different directions. For example, your screen can go up and down + left and right. This means it is two dimensional. Something that is two dimensions is called a Planar.

3 dimensions

Something is three dimensions when it can not only move up and down + left and right, but it can also go in and out of something. This is sort of like our three-dimensional space.

Hope this helped!

Source: Video
• More than three dimensions? What would that look like?
• Imagine a 3D sphere and a 2D 'plane segment'. Now imagine the sphere passes through the plane segment and ask 'What does that sphere look like to the plane?' What we perceive as time is the 4th dimension expressing itself through the 3rd (or should I say we are 5th dimension objects passing through the fourth), and so on up the dimensions.
• Is advanced geometry the same as high school geometry? I'm taking advanced geometry this semester and just want to make sure.
• Hey, purpleox! I would encourage you to contact your school/district to address your questions and concerns before your school starts instead of posting here on Khan Academy (there are many people who have different experiences that may not exactly relate to yours.)

Anyways, here's a quote that I hope helps you on your learning journey.

“When the going gets tough, put one foot in front of the other and just keep going. Don’t give up.”
― Roy T. Bennett, The Light in the Heart

Have a wonderful day (and school year!)