If you're seeing this message, it means we're having trouble loading external resources for Khan Academy.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Watch the entire video

Lines, line segments, and rays

Difference between lines, line segments, and rays

Lines, line segments, and rays

Discussion and questions for this video
would an infinite line and an infinite ray be equally long? That's my question.
Yes. It's a tricky concept because it feels like an infinitely long ray is only half as long as an infinitely long line. But technically, half of infinity is still infinity.
What is the difference of a ray line segment and a line?
A line segment connects two points and stops at those two points.
A ray starts at a point and continues forever in one direction.
A line continues forever in both directions.
So then what is the point of infinite ray and infinite line?
A ray begins at a point and then continues on in one direction like the rays of the sun. An infinite line goes off in both directions. It has no beginning and no ending.... much like a highway has many points you can stop along the way but seems to continue on and on in both directions.
A segment(when talking about a line) is only part of a line. Like he said, a line goes on forever but a line segment doesn't. ♥
What if the ray was coming from both sides like this:
Would this be a ray or a line?
what is the difference between a ray segment and a line?
A ray has one endpoint and travels in one direction, while a line is the total opposite of a line segment, having no endpoints and travelling in 2 directions, going into infinity.
what are skew lines
In other words, you can only have skew lines in 3 or more dimensions, because in 2D plane geometry, lines are either parallel or intersecting, but not both and not neither.
at 1:49 in the video, it said that a ray had a well defined starting point? What did they mean? Did they mean the little arrow on the end?
A line is a series of points which keep growing from both ends. A line segment is a portion of line with two end points. A ray is a line which has got one end point. The open end point is infinite.
I don't think so! Rays only have one end point, line segments have two end points in which do not go in either direction, and Rays on the other hand go on forever in only one direction because Rays only have one end point!
can a line also be a ray?
Lines cannot be rays.
Line: Extends forever in two directions
Ray: Extends forever in one direction and has one endpoint
Line Segment: Has two endpoints and doesn't extend forever
if a a ray is infinte but with a starting point does it never come back onto itself? essntialy then turning itself into a line?
Well that cant happen because a ray goes on in one direction for ever, and a line goes on for 2 directions forever.
how can you remember the segments more easily
A segment is not infinite and has 2 endpoints.
At 2:33 they say that the figure is a line. But two endpoints are given, so it doesn't make sense for it to be a line and not a line segment.
It's useful to focus on the arrows - at both ends - that tells us, "YES" it can be a line.
(In the video around 1:00 minute, when Sal sketched a line... I think he should sketch two points along the line, however, he was busy emphasizing those arrows at each end to show it is a line.)
It is good that you saw that it seems to represent a line segment, because the two points there, show a segment within the line. (If the question was, can this represent a LINE SEGMENT? it would be right to say, YES.)
A LINE is defined by two points (where any point has a location) and the line does NOT STOP at either point, so two "arrows" in the sketch always means "this is a line."
Two points A and C and the sketch extends with lines and arrows away from both of two points, can correctly be identified as line segment AC, ray AC, ray CA, and line AC (or line CA).
nowadays why do people call line segments ,lines
In a conversation outside a math classroom, "line" is descriptive enough, But math users like to be precise.
Why would there be line segments inside a line? I thought It was only part of a line and it wouldn't be inside
If there are four points on a line, there are nine different segments. Think of each little space between the points as a segment by itself. All the other segments create other segments, and each single segment can be used as many times as the patterns require. It's the same basic principle as those "Jim has three shirts and three pairs of pants. How many combinations are there?' questions.

Hope this helps and I didn't confuse you!
would the steps to finding the answer to these problems, always be the same?
Is there such thing as a acute line segment
No, "acute" only applies to angles that are smaller than 90 degrees.
Even though I think I know the answer,why can't an infinite line and an infinite ray be the same?
So Is the universe a ray,line or line segment?
The universe is a plane. Like a plane, it goes on forever and ever.
So what is the difference between a ray and a line?
A line goes on forever in both directions. A ray has a starting point, then goes on forever in one direction, kind of like a flashlight beam.
What are the similarities and differences between a line and a line segment?
A line segment is a portion of a line that has a finite length.
Does a line have to have arrows at the end?
no:) as long as you don't put endpoints on it, you can leave the sides open.
^see that still counts as a line cause it has no endpoints.
wouldnt two rays be equally long
Are lines and rays associated?
Sal Just a side note, if you play the video," recognizing lines"in 4th grade, and add captions, it talks about beer and counting Koreans, I think the computer takes what your saying and mishears it.
would an infinite line and an infinite ray be equally long?
Infinity is infinity. So yes, they would be equally long. Imagine that a ray has a length of infinity. Then imagine a line that has a length of infinity. In theory, they both have lengths of infinity and the line cannot be longer than the ray. Now, if 'infinity' were a normal number, then the line would be twice as long as the ray. But, as I said, infinity is infinity, so in both cases they are extending forever. Hope this helps!
P.S.: This is fun to think about, isn't it?!
how much time does it take to make one of your supreme videos
What happens if you tell the difference?
what's the difference between an infinite ray and a ray?
If a ray went infinitely around the Earth, would the end of the ray that keeps going meet up with the other end and become a line?
What is described in the question above, is a curve, and it is not in a single direction like a ray has to be in a single direction. A ray can not curve around Earth, a ray can start at Earth and extend to space. Curves, are never rays, or lines, in geometry definitions. A ray can never meet the start point. Also, a line can not be defined as in the question, as something that goes around and meets the starting point, a line can only go infinitely along in directions away from one point, never meeting up "end to end."
Your question is so important because it hints that you are thinking about the idea of a "curved ray" (or "curved line") and you will have a better time with calculus, because you are thinking about "curved lines".
To be certain about solving geometry problems it's important to be certain that a ray is described by an start-point and a single direction, not a curve. A general example is, think of the ray of light (like from a laser) it's got a start point, and goes in a single direction - a ray of light.
(A "curved line" is - mathematically - made of teeny segments connected point to point. Here you are entering into calculus, where the teeny segments get cut up teenier and teenier.)
Hope that helps.
Can someone make this a little more simple please?
What is the difference between a line and line segment? (Curious)
A line goes out to infinity in both directions. A line segment ends and has a finite length.
What If I were to draw a line around the circumference of a sphere for example, the equator. Do I have a line or a line segment?
If a line segment has a starting point and an ending point, and a ray has a starting point but no ending point, and a line has no starting point or ending point, what would you call something with an ending point but no starting point?
The so-called "Ending-point" would be thought of as the starting point. Around 1:40 ish, Sal explains the ray, which your question directly relates to. A ray does have a starting point, but no ending point. Ray = infinite. Good question, though! Hope this helps!
why did they come up with signs to represent rays, line segments, and lines?
what does collinear mean?
Points that are in order to form a line
can an acute angle go on forever
Yes, because any angle is made up of a series of lines and if you watched this video you know what a line is...so it does go on forever...that is a great question though...
what is the difference between a line segment and a line
A line segment is a segment of a line, also known as a section of a line. They are made up of two points on each end of the segment, with no arrows. A line is a straight mark that consists of two points aswell, but they always have to have the arrows on the ends of the line. A line can go on virtually infinately,since it has those little arrows on the end.
Grocery stores go on for EVER for KIDS!
no they don,t grocery stores start at one end and end at another.
that is called a segment line
Sal mentions the fact that very few objects/situations in our lives go on forever and it got me thinking. Say planet Earth has a single road that goes around the circumference of the planet. Does the road go on forever? I appreciate that if you were to drive on it then it would appear to go on forever but ultimately it's just looping over the same locations.

I guess I'm asking if there is a difference between me shooting a rocket into space that will travel in one direction forever (and constantly going through new space) as oppose to a loop which repeats the same physical space.

Just in case I get attacked by moderators (;-)) my link is to lines and infinity as mentioned in the video.
How do u find the ray segment of a line
I'm not sure what your question means. Could you explain further?
What does congruent mean?
If infinity is forever does that mean half infinity is infinite to
The short answer is no, because infinity isn't really a number
The long answer is in set theory. If you have a set of all the rational numbers (From -infinity through +infinity (Excluding irrationals)) and a set of all the REAL numbers. From (+infinity to -infinity) the two sets are both infinite but different sizes.

As an interesting aside, the set of positive numbers is the same size as the set of negative and positive numbers, which can be shown by a proof that is rather complicated and long to type out here.
would an infinite line and an infinite ray be equally long? That's my question.
Yes, a line and a ray are the same length -- both are infinite. I know that common sense would appear to suggest that the line is twice as long, but they are indeed the same length -- the line's infinity is not bigger than the ray's infinity!
Where can you find a video about collinear and coplanar lines?
"Coplanar" just means "on the same plane". If you have a bunch of rays and lines, but they're all on the same plane, they're "coplanar".
Wait,so a line is a endless line?
Correct. A line never ends. You might say "but we see lines end all the time when drawing!" well, those are line _segments_ and not lines. In math, they're signified using dots on either side. Lines are represented with arrows pointing outwards in the directions they're going.
Is there an easier way to memorize which line, line segment, and ray is which? Like some sort of handy trick...
What is the difference of a ray line segment and a line?
so how do i know wat the difference is from fay and linesegment
A ray has an arrow on one end and a line stops at two points. The store on the ray represents it going on forever in that direction.

I hope this helps!
Why is a line segment with a ray on one of the points called a ray? Why not call it a line segment?
A line segment has length, which is different from a ray which goes off forever in one direction or a line which goes off forever in both directions.
What is an example of a ray?
A ray is line going off and never stopping like an arrow.
what is the difference between a line and a ray?
A line has no endpoints, it goes on forever in two directions.
A ray has one endpoint (which you may think of as its beginning) and then goes on forever in one direction.
A line segment has two endpoints, it does not go on forever at all. Of the three, only the line segment has a finite length.

All three of these, the line, the ray, and the line segment, are perfectly straight without any curving at all.
can a ray go on forever
Yes! Actually a ray can go on forever even though it is going in only one direction.
What is the difference between rays, lines and line segments?
So is a circle a line or a ray?
Neither. Lines and rays are straight, and a circle is not.
how to take a test in this stuff
Can a ray be named after a single point(origin) or is it necessary that it should pass through another point, also if a ray is passing through three points or more
for example : a ray starting at point A passing through point B further through point C and the ray is named as Ray AB can it also be called Ray BC?
A ray needs to have two points. Either name is acceptable.
hello, I am a 14yr old high school student who needs help in geometry.my questions, if RS=15 and ST=9, then RS=
If RS=15 then RS=15.
Is there a way where the line on a ray can be in a specific way? like left or right?
yes, if the arrow is on the left side, it goes infinently to the left, if the arrow is on the right, it goes infinently right, and so on.
How do you find intersections? I forgot...
In this video you explicity state that lines and rays use arrows to denote they continue in both or one direction, respectively. Is this a universal notation used in Geometry? In the exercise following this video, none of the lines or rays have arrows on them and the correct answers do not use the same conventions as demonstrated in this vido.
If rays have only one direction then why is the starting point of a ray called an "endpoint" when it should be called "startpoint" ? lol. The drawing line exercise after this video is so confusing when they use that terminology because ray AB is not the same as ray BA even though they have the same points.
can a line be a ray
But you could have a line and name a certain part of it as a ray:
Pretend that's a line with two points on it, A and B.
You could call it "line AB" that goes on forever in both directions, but you could also say that there are rays in there, like "ray AB" that starts at A, goes through B, and then goes on forever in one direction.
how do you identify rays and lines . they both look same
Lines extend forever in both directions. It looks something like this:
The arrows on each ends indicate that the line keeps on going in both directions, infinitely.

A ray starts off at a point but extends forever in the other direction, or has no end point. Something like this:
The dot on the left indicates the ray's starting point, and the arrow shows that the ray extends forever in that direction.
Why on a line segment, why are there the two arrows as if it's still calling it a line if the two circles weren't there?
What would you use rays and lines for?
so a line segment can go forever, huh? intresting!
No a line segment has a starting and ending point, but a line can go on forever yes
At 0:36 Sal said line segment what does segment mean?
So what does collinear, coplaner mean? Isnt there a video for that? i cant find it.
Can someone please help?

~Darshi Ambani
Collinear means on the same line, and coplanar means on the same plane (:
In the quiz he was taking he said the line with a dot and arrow on each side was a line not a line segment. does this mean that even if a line has two dots on it as long as it has two arrows aswell it is classed as a line and not a line segment?
wouldn't lines of longitude and latitude be examples of a line that never ends since it goes around the world or would lines that over lap themselves have a stopping point?
um.. actually lines of latitude will not be a line it will be a circle because the definition of a line is a figure made of a series of points linked in a sequence and has no ending. Since latitudes have a ending they cannot be called a line.(line technically has no ending and the thing that is a line with a ending is called a line segment)
E is the midpoint of line DF
DE=2x-3, EF=5x-24. Solve for x
If E is the midpoint then DE= EF so then 2x-3=5x-24.
Subtract 2x on both sides of the equation to end up with:
Add 24 to both sides of the equation to end up with:
Divide 3 to both sides of the equation to end up with:
Make sure to check by plugging in 7 to all x's.
Hoped this helped! :)
so hard how do you this i dont really get it.
So do lines go on forever???
Yeppers, He says it in the video (:
So pay more attention... jkjkjjk!
Im not a meany. lolz
Are rays and segments almost the same?
Hmmm..."almost" in definitions is not something mathematicians use...we're kinda anal retentive that way!
BUT, I see where you are coming from. A segment has 2 end points and a finite length. A ray has 1 endpoint and infinite length.
helping daughter with hon. geom...how do I find the length of the segment when given J(-2,4) and K(1,3) for example? Formula?
You would use the distance formula. The formula is the square root of ( (x1-x2)^2-(y1-y2)^2 ), where y1 and x1 are the x and y coordinates of the same point.
so its neat and you can read it
i am not able to understand coplanar points and collinear points and their respective problems
is the line segment AB = to AB
I love this video but can a line go and retrace it's steps?
My math textbook asks "In how many points can two distinct lines at the most intersect?" and the answer says one. Wouldn't the answer be infinite because a line passes through infinite points?
so if line and line segments and rays and so measuring segment ------------> but the ray to the line
Why in the quiz he was taking it had dots (indicating a beginning and ending ) and arrows (showing it goes on forever) ? 2:29 . How would I know whether it is a line segment line or ray?
why always a intersecting line or a parllel line be in can be always called in line?
why cant we call it in a line segment? for ex: parllel linesegment?