High school geometry
Sal constructs a regular hexagon that is inscribed inside a given circle using compass and straightedge. Created by Sal Khan.
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- I have heard of equilateral, equiangular, and regular. Once you get to pentagons and up they don't have the same meaning.
You can have a pentagon with all sides the same length but unequal angles.
You can have a pentagon with all angles the same but unequal side lengths.
You can have a pentagon that is both equilateral and equiangular which is a regular pentagon.
But this seems counter intuitive to me. How can a polygon be equilateral and not equiangular?(23 votes)
- We have equilateral quadrilaterals that are not necessarily equiangular (they're called rhombuses)
We have equiangular quadrilaterals that are not necessarily equilateral (they're called rectangles).
It is only with triangles that a polygon must be equilateral if it is equiangular and must be equiangular if it is equilateral. All other polygons can be equiangular, equilateral, or both.
I suggest you just make a stick model of an equilateral polygon, notice you can physically alter angles and still be able to fit them together into a polygon.(13 votes)
- What does it mean to be a regular hexagon? I can't find a video explaining what the meaning of "regular" is in terms of polygons.(6 votes)
- In geometry, a 'regular' polygon is simply a polygon that is both equilateral and equiangular. For example, a regular triangle would have all sides that are equal, and each interior angle would be 60 degrees.(12 votes)
- i understand why the four sides of the hexagon who serve as radii for the additional circles are equal sides. but how do we know that the 2 remaining side (upper and lower) are equal to the rest of the sides?(7 votes)
- If you were connect the center of the circle to both of the top intersections, you would form to equilateral triangles. A quick calculation tells you the angle between those radii is 60 degrees. Since the radii are equal, you have an isosceles triangle with a vertex of 60 degrees, meaning you have an equilateral triangle. Therefore the segment connecting the top intersections (and the one on the bottom) is equal to the radius of your circle.(8 votes)
- Is there any way to construct a regular octagon inscribed in a circle?(4 votes)
- For an octagon, you basically just need to divide the circumference into eight equal pieces. One possible method (though there's a few ways to do it) is:
1. Construct a diameter.
2. Construct the perpendicular diameter (i.e. the perpendicular bisector of the first diameter).
3. Bisect one of the right angles, and draw another diameter - that gives you four arcs subtended by 45°, two on each side of the circle.
4. Now bisect the other right angle, and draw another dimeter - that's the other four arcs.
5. Now just join up all the points where the diameters intersect the circle.(6 votes)
- What was the point of that initial long line through the circle? It never seemed to really get used...(2 votes)
- I think that was simply to make it easier to decide where to put the the two centers of the circles; after all, they have to be directly across from one another, one center at each point where line and circle intersect.(2 votes)
- Is there a video that can help me with a question I have? I have a question about an assignment and don't really know how to do it. It says "Construct line j through D with j ⊥ l."(1 vote)
- Is there a video where SAL shows s how to construct a pair of parallel lines??
I f so please reply quickly...need an answer before the end of this week...gotto prepare for an exam...(1 vote)
- There are a couple of sections related to parallel lines. Pick the one that helps:
FYI... Use the search option to find topics (click on magnifying glass at the top of any screen). You could have found these yourself and saved 3 days of waiting.(2 votes)
- I am interested in how to construct a regular pentagon in a circle, our geometry teacher taught me how to do it, but I did not know why it work like that(1 vote)
- do you have to get the exact answer for the question to be correct(1 vote)
- In a real problem the answer must look like a hexagon and have six sides, but yeah the answer needs to be as close as possible(1 vote)
Construct a regular hexagon inscribed inside the circle. So what I'm going to do, first, I'm going to draw a diameter of the circle. And actually, I'm going to go beyond the diameter of the circle. I'm just going to draw a line that goes through the center of the circle and just keeps on going. And I'm going to make it flat, so it goes directly through, so this one right over here, goes directly through the center. And now, what I'm going to do is I'm going to construct a circle that's the exact same dimensions of this circle that's already been drawn. So let me put this one right over here. And let me make it the same dimensions. And now, what I'm going to do is I'm going to move it over, so that this new circle intersects the center of the old circle. And these circles are the same size. So notice, this center intersects the old circle. And the new circle itself intersects the center of the old circle. Now, the reason why this is interesting, we already know that this distance, the distance between these two centers, this is equal to a radius. We also know-- we have a straight edge-- we also know that this distance right over here is equal to a radius. It's equal to the radius of our new circle right over here. We also know that this distance right over here is equal to a radius of our old circle. And that they both have the same radius. So this is a radius between these two points. Between these two points is a radius. And then, in between these two points is a radius. So now, I have constructed an equilateral triangle. And essentially, you have to just do this six times. And then I'm going to have a hexagon inscribed inside the circle. Let me do it again. So we'll go from here to here. This is a radius of my new circle, which is the same as the radius of the old circle. And I could go from here to here. That's the radius of my old circle. So I have another equilateral triangle. Radius, radius, radius. Another equilateral triangle. I've just got to do this, I have to do this four more times. So let me go to my original. Let's see, let me make sure I can-- well, it's actually going to be hard for me to-- let me just add another circle here, to do it on the other side. So if I put the center of it right-- I want to move it a little bit, right over-- I want to make it the same size. So oh, that's close enough. And let's see, that looks pretty close. That's the same size. Now, let me move it over here. Now, I want this center to be on the circle, right like that. And now, I'm ready to draw some more equilateral triangles. And really, I don't have to even draw the inside of it. Now, I see my six vertices for my hexagon, here, here, here, here, here, here. And I think you're satisfied now that you could break this up into six equilateral triangles. So let's do that. So this would be the base of one of those equilateral triangles. And actually, let me move these. Let me move this one out of the way. I can move this one right over here. Because I really just care about the hexagon itself. And I can move this right over here. But we know that these are all the lengths of the radius anyway. Actually, I'm not even having to change the length there. Then, I have to just connect one more right down here. So let me add another straight edge, connect those two points. And I would have done it. I would have constructed my regular hexagon inscribed in the circle.