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The angle at the heptagon’s centre is equal to
360°/7 = 51°,42
The heptagon side GH is equal to the radius OH * 2 sin (51°,42/2)
If the radius =1, sin (51°,42/2) = 0,4338 -> GH = 0,867
On the same circle create the equilateral triangle ABC
We notice that BD = radius * cos 30°, or, radius * sin 60°, or sin 60° = cos 30° = 0,866 = BD
The half side of the inscribed equilateral triangle is a very approximate value of the side of the inscribed heptagon in the same circle.