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If I know how long one side of a regular hexagon is, what's the formula to calculate the radius of a circle inscribed inside it?

Illustration:

in-circle in a hexagon

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  • $\begingroup$ Is the hexagon is regular? $\endgroup$ – kennytm May 1 '16 at 15:33
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    $\begingroup$ Hint: you can divide a regular hexagon into six equilateral triangles. $\endgroup$ – Wojowu May 1 '16 at 15:35
  • $\begingroup$ yes all sides are equal. but if I make a circle that has the radius equal to a hexagon side it doesn't fit the hexagon $\endgroup$ – puffy roxxy May 1 '16 at 15:35
  • $\begingroup$ It depends on what is given. E.g.: Radius^2 + (half length of one side of the hexgon)^2 = (half length of one side of the white square)^2 . $\endgroup$ – user90369 May 1 '16 at 15:37
  • $\begingroup$ Hint: what is the height, not side, of an equilateral triangle. The hexagon divides into six equilateral triangle. The radius of the circle is not the side of this equilateral triangle; it is the height of the equilateral triangle. $\endgroup$ – fleablood May 1 '16 at 15:41
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Label the center of the circle. Draw six lines from the the center to the circle to the vertices of the hexagon. (These lines will be longer than the radius.) This will divide the circle into six triangles.

Question for you: Tell me every thing you can about these triangles. In particular, what are the lengths of the lines from the center?

Now draw six radii of the circle to the six edges of the hexagon. Along with the six "spokes" before you have divided the hexagon into twelve triangles.

Question for you: tell me every thing you can about these triangles. In particular:

are they congruent to each other?

what are the angles of these triangles?

What are the lengths of the sides of these triangles?

And from there I will ask you these two questions: What is the radius of the circle? and, what is the formula for the area of the circle.

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  • $\begingroup$ Like the answer because you didn't do his work for him! $\endgroup$ – Torrien Oct 12 '16 at 12:50
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The radius equals the height of the equilateral triangles of side $s$.

By Pythagoras,

$$h^2+\left(\frac s2\right)^2=s^2$$ so that

$$h=\frac{\sqrt 3}2s.$$

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Draw the six isosceles triangles.

Divide each of these triangles into two right angled triangles.

Then you have

$s = 2x = 2 (r \sin \theta)$

where $r$ is the radius of the circle, $\theta$ is the top angle in the right angled triangles and there are in total $12$ of these triangles so its easy to figure out $\theta$. $x$ is the short side in these right angled triangles and $s$ is of course the outer side in the isosceles triangles, i.e. the side length you say you know.

Hence the formula for the radius is

$$r = \frac{s}{2 \sin \theta}$$

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A regular Hexagon can be split into $6$ equilateral triangles. Since the inscribed circle is tangent to the side lengths of the Hexagon, we can draw a height from the center of the circle to the side length of the Hexagon.

Using the $30-60-90$ rule, the height is $\frac {x\sqrt{3}}{2}$ with a Hexagon with a side length of $x$ units.

So the radius of the circle is $\frac {x\sqrt{3}}{2}$ with $x$ as a side length of the Hexagon.

*NOTE: This is only true when the Hexagon is a regular Hexagon!

And for the area of the circle, just use the formula for the area of a circle ($A=\pi r^2$) where $r$ is the radius.

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