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Geometric constructions: triangle-inscribing circle

Sal constructs a circle that inscribes a given triangle using compass and straightedge. Created by Sal Khan.

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  • hopper cool style avatar for user Dhaval Furia
    The question asks to construct a circle inscribing the triangle.
    So doesn't that mean the triangle should be inside the circle ?
    The opposite goes for the next VDO : Constructing circumscribing circle
    (22 votes)
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  • blobby green style avatar for user Sherwood Botsford
    The use of the 4 headed arrow is sub-optimal. Much better would be a hollow circle with short horizontal and vertical lines (about 1/2 radius) pointing inward so you could see exactly where you are placing the dot. Further: The drawing should be closer to the 'ideal' line and point.

    This is important as multi-stage constructions have accumulated errors. E.g. in the video on an inscribed circle the speaker demos how to do it, and despite exercising reasonable care, his circle doesn't quite touch one side. This both introduces the idea that sloppy workmanship is tolerable, and introduces doubt as to the validity of the construction.

    Also: There is no explanation as to why the construction is valid. Why is the intersection of two bisectors the centre of the circle? By what magic is this the right place for the third side.

    The proof is not difficult, although it has many steps:

    Bisect the angle.
    Pick a point on the bisector.
    From that point construct perpendiculars through that point to each of the two sides of the angle.

    Show that the two triangles formed are congruent.

    Since the point is arbitrary, it means that any point on the bisector is equidistant from both sides of the triangle.

    Repeat for another angle.

    Repeat the construction from the intersection to all sides. One of the perpendiculars will be a side of two different triangles. Equality is transitive so if A=B and B=C then A=C so all three lengths are equal.
    (6 votes)
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    • piceratops ultimate style avatar for user Amilia
      I agree that this does introduce the idea that "not quite", or "almost" is tolerable, but sometimes it is. The point here is to teach you how to do it, not to be a perfectionist. Khan academy usually teaches one method, the one they think is best. There are many different methods and proofs you can find online. If you can do a proof yourself, why does someone need to show you? Good question Sherwood.
      (0 votes)
  • starky ultimate style avatar for user Swara Patil
    How does Sal know to take the two circles and place them like that? How does he know it will bisect that angle?
    (2 votes)
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    • mr pink green style avatar for user David Severin
      Sal kind of skips the normal first step of starting at a vertex and drawing a circle, then drawing two congruent circles whose intersection would be equidistant from the two points. Sal avoids the first step by drawing two congruent circles whose radii are equidistant from the vertex. Does this help, or you need more explanation?
      (1 vote)
  • aqualine ultimate style avatar for user SD
    Why doesn't Sal use the perpendicular bisector method, by putting a compass on either vertex, to create the bisector?
    (2 votes)
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  • blobby green style avatar for user MAgicForealz.01yeahyeah
    What can j be doing wrong? I do all the steps (double checked ) and yet my circle is never the right size.
    (2 votes)
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  • spunky sam red style avatar for user Marvin Cohen
    Can you construct a circle that is inscribed in a square?
    (1 vote)
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  • blobby green style avatar for user Arbaaz Ibrahim
    How do you construct a circle in a triangle using a compass?
    (1 vote)
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  • blobby green style avatar for user Arbaaz Ibrahim
    When the question asks to construct a circle inscribing the triangle, does it want the largest possible circle, or a circle of any size?
    (1 vote)
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  • blobby green style avatar for user krffaraon
    I haven't watched any videos about the intersection of triangle/s angle bisectors in High School Geometry. Where can I find the video proof?
    (1 vote)
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  • blobby green style avatar for user mohamad
    how do we prove it using congruent triangles?
    (0 votes)
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Video transcript

Construct a circle inscribing the triangle. So this would be a circle that's inside this triangle, where each of the sides of the triangle are tangents to the circle. And probably the easiest way to think about it is the center of that circle is going to be at the incenter of the triangle. Now what is the incenter of the triangle? The incenter of the triangle is the intersection of the angle bisectors. So if I were to make a line that perfectly splits an angle in two-- so I'm eyeballing it right over here-- this would be an angle bisector. But to be a little bit more precise about angle bisectors, I could actually use a compass. So let me make this a little bit smaller. And what I can do is I could put this, the center of this circle, on one of the sides of this angle right over here. Now let me get another circle. And I want to make it the same size. So let me center it there. I want to make it the exact same size. And now let me put it on the other one, on the other side of this angle. I'll put it right over here. And I want to put it so that the center of the circle is on the other side of the angle, and that the circle itself, or the vertex, sits on the circle itself. And what this does is I can now look at the intersection of this point, the vertex, and this point, and that's going to be the angle bisector. So let me go-- I'm going to go through there and I'm going to go through there. Now let me move these circles over to here, so I can take the angle bisector of this side as well. So I can put this one over here. And I could put this one-- let's see, I want to be on the side of the angle. And I want the circle to go right through the vertex. Now let me add another straight edge here. So I want to go through this point and I want to bisect the angle, go right through the other point of intersection of these two circles. Now let me get rid of one of these two circles. I don't need that anymore. And let me use this one to actually construct the circle inscribing the triangle. So I'm going to put it at the center right over there. Actually, this one's already pretty close in terms of dimensions. And with this tool, you don't have to be 100% precise. It has some margin for error. And so let's just go with this. This actually should be touching. But this has some margin for error. Let's see if this was good enough. It was.