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Euler line

The magic and mystery of the Euler Line. Created by Sal Khan.

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  • purple pi purple style avatar for user gilbert
    What is the 9 point circle?
    (16 votes)
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    • aqualine seed style avatar for user Sandra Dauber
      @ bodickstein whatever wikipedia is a great source for all sorts of information thus liberating students from only learning from textbooks or relying completely on teachers. And even though a lot of people don't like Wikipedia because anyone has the ability to edit it, you'll see that if something get's put in that shouldn't be there the staff immediately corrects this.
      (3 votes)
  • mr pants teal style avatar for user hkapur97
    WOW!
    This video was awesome!
    What else did this guy Euler do?
    He seems to be a really smart person!
    (14 votes)
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  • spunky sam blue style avatar for user Samir Gunic
    Can you show us a video on how to draw a triangle with the Euler line? I sure could use that because my every attempt fails, when I am using a straight edge ruler and protractor. It just doesn't add up... some line is always off the center, or I get an arc as opposed to a line, if there is such a thing as Euler arc!... Of course if I draw it freehand I can make it look any way I like, I can even make it look like in this presentation and say it's true.

    And sometimes the centroid or the orthocenter is outside my triangle. What type of triangle is best for demonstrating this relationship with Euler line? Isosceles triangle? So that all the concurrencies (centroid, circumcenter, orthocenter) sit within the triangle? What angles would you use for that triangle?...
    (3 votes)
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  • male robot hal style avatar for user aditya.sharan
    While graphing triangles, what do we mean when we say that the triangle is centred at a point, say (2,3)? Is this point the centroid, circumcentre, orthocentre or the 9-point centre?
    (4 votes)
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    • orange juice squid orange style avatar for user Troy Cook
      I was always taught the center refers to where the median lines meet. Later I was introduced to the centroid which is the same as the center. If you think about this intuitively, it is the center of the area of the triangle and its center of mass (if it had a consistent thickness). You can cut out any triangle and balance it on its center (centroid). When you divide the triangle into three smaller triangles using the centroid as the common vertex, all smaller triangles have the same area.
      (3 votes)
  • blobby green style avatar for user BrianDGlen11232
    will I ever use this in my life because I think not
    (2 votes)
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  • piceratops ultimate style avatar for user Paige VanGieson
    Neat stuff, but I'm curious about its applications. Does the Euler line find use in science, engineering, or any other discipline? Excepting mathematics, of course =)
    (2 votes)
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  • male robot hal style avatar for user Ryan Clark
    Is the orthocenter outside of the triangle for an obtuse triangle?
    (1 vote)
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  • blobby green style avatar for user y.ososami
    This was very specific but is it possible to get the Euder line on an angle bisector, if possible how?
    (1 vote)
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  • spunky sam red style avatar for user Satyam Sharma
    Can you please make videos on Euler's great achievements, and how he accomplished them? I have studied and understood only a fraction of Euler's work, and I have learned more from one video that Euler solved or accomplished than a series of videos. Thanks, Dylan.
    (1 vote)
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  • leaf green style avatar for user loic.alini
    Why isn't the incenter on Euler's line?
    (1 vote)
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Video transcript

So I've drawn an arbitrary triangle over here that we will assume is not an equilateral triangle. And let's draw some of the interesting properties of this triangle. So, let's first start with the perpendicular bisectors. So let's say this is the midpoint of this side right over here. So-- draw a little bit closer to the midpoint-- looks like right about there. So this length is equal to this length right over here. And so let me draw a perpendicular bisector. So perpendicular bisector would look something like that for that side. Now let me do it on this side. So just trying to eyeball it, this looks roughly like the midpoint. So this length is going to be equal to this length, and then once again, let me draw a perpendicular bisector, so it's going to go at a 90 degree angle. I can draw a straighter line than that. So that looks pretty good. And it is perpendicular. And then you can imagine I will do it with this side, with side AB. That looks like the midpoint. This length is going to be equal to that length right over there. And then if we draw a perpendicular-- my best attempt to draw this-- it looks something like that. And we've already learned that these three lines will definitely intersect in a unique point right over there, which we call the circumcenter. So let me write it. So this right over there is our circumcenter. This is nothing new. Now let's draw the medians. So, the medians go from each of the vertices to the midpoint of the opposite side. So let me draw. So this is a median, this is a median, this is a median, and then this is-- and all the errors in my drawing will start to become apparent, now that I'm trying to do multiple things with it. But we know all of the medians intersect at the centroid. So this right over here is the centroid. Now let's draw the altitudes of this triangle. So let's draw the altitude. So I'm going to drop a perpendicular right over here. This is one altitude. I will draw another altitude. So remember, it's going to go down like this. Draw another altitude that looks something like that. And then I can draw another altitude from this side, and we know it's going to intersect at this unique point right over here. So I'll try to draw it as neatly as I can. And this is going to be perpendicular, although the way I've drawn it, it doesn't necessarily look that way. And we know that this thing right over here is called an orthocenter. Now the whole reason why I'm doing this, and this is frankly kind of a mind-boggling idea. It's neat enough that each of these points even exists, that three altitudes will intersect in one point, that the three medians intersect in one point, and they have all these neat properties, and that the three perpendicular bisectors intersect in one point. That by itself is neat enough. But what's really neat-- and it's maybe not completely apparent by the way I drew it, because I just drew it free hand-- is that these three points will definitely be on the same line. If this was an equilateral triangle, they would actually be the same point. But for any other triangle there'll be different points, and they will be on the same line. Which is just kind of crazy, because obviously two points define a line, but three points being on the same line is kind of a-- it seems like a very unlikely thing, if you were to just guess it. And I'm going to try to draw it as best as I can. If you were to draw this with a straight edge ruler, it would come out a lot cleaner. But all three points sit on this unique line that, it seems something special or magical about it. And because it's something special or magical, there's one famous mathematician who tends to get all of the most special and mathematical things named after him, because he's the guy who's really explored these things. And we call this the Euler line. And I say that Leonhard Euler-- that looks like ENTER. I say that he gets all of the cool and magical mystical things named after him, because he's also responsible for Euler's identity, which is e to the i pi is equal to negative 1. And we proved this in the calculus playlist, and if none of this makes any sense to you, don't worry. We're only in geometry right now. But he gets to have all the-- This is a magical thing because e comes out of compound interest, and growth and decay, exponential growth and decay. i squared is negative 1-- seems a very bizarre imaginary number. Pi is a ratio of the circumference of a circle to its diameter. And negative 1 is, well, negative 1. And so Euler showed that there's a lot of reason to believe that these four numbers that come from all of these different weird realities of the world are connected in this very tight way. And he also showed that these special points are all on the same line, which tells us something kind of crazy and mystical about our reality. And if that is not enough for you, if you were to take the midpoint on the Euler line between the orthocenter-- I'm not going to prove it here, I'm just going to give you a little bit of a tidbit of it. If you take the midpoint between the orthocenter and the circumcenter-- so let's take this. So I'm going to try to look at it roughly. So it looks like it's right about there. So this length is going to be equal to this length right over here. This point that sits on the Euler line is going to be the center of something called the nine-point circle, which intersects this triangle at nine points. And we'll see this kind of nine interesting points. So let me label that as well. So it's cool enough that these three special points are on the Euler line, but there's actually four special points. And actually, there's a few more than that are kind of interesting. So that orange point right over there is the center of the nine-point circle. And maybe I'll do another video, another video just on that. So anyway, hopefully you found that kind of interesting.