I can't believe that on Physics Forums, of all places, two and a half pages worth of posters managed to miss this on the first page.
crocque said:
"Inside a regular inscribed hexagon, the radius of the circle is equal to the sides of the hexagon".
Picking at the use of "inside" would have made sense, but he correctly stated that the hexagon was inscribed. Barking up the wrong tree, if you ask me.
The geometric proof is simple and can be done informally as follows:
A regular hexagon is an regular n-gon with even n. That means that:
1. You can connect every pair of opposite vertices using a diagonal, and all these diagonals will intersect at the same point.
2. You can inscribe it within a circle with all vertices of the n-gon touching the circle.
3. The internal angle between two sides of a regular polygon is bisected by the diagonal.
With these preliminaries out of the way, draw the three diagonals between the three pairs of opposite vertices.
It isn't hard to see that the internal angles are 120 degrees, and the bisecting diagonal turns this into two angles of 60 degrees, side-by-side around the bisector.
From 1 we have that the hexagon has been split into six triangles. From 3 we have that, for each of these triangles, two angles are 60 degrees.
Thus it holds necessarily that these triangles are equilateral. Next we use the lemma that the radius of the circle is equal to the distance from the center of the inscribed polygon to any vertex thereof. My proof is as follows:
We note that, from 2, we know that the vertices of the hexagon all lie on the circle, and from our knowledge that the triangles are equal, we see that each of the six vertices is equidistant from the hexagon's center (described by the intersection of the diagonals).
Now we look at this from the perspective of the circle in which the hexagon is inscribed. Every point on the circle is equidistant from one particular point within the circle; this unique point is the center and the unique distance the radius.
As a finite subset of the infinite set of points on the circumference of the circle, there is a set of six points which form the vertices of the hexagon. These too must be equidistant from the center of the circle.
However, both their distance from the hexagon's center and the hexagon's center itself are known, and the distance and point must be unique. Thus we can conclude that the length of one side of the equilateral triangle is equal to the radius and the center coincides with the center of the circle.
Given this it is trivial to prove that, since two sides of any of the equilateral triangles are radii of the circle, and the third (due to our construction) is one of the sides of the hexagon, the radius of the circle is equal to any side of the hexagon. QED