I was born 51 years ago today. To mark the occasion, here’s a 51-pointed star. It’s made of {17/7} heptadecagrams repeated 3 times, and with their edges extended as lines, in three different colors. This works because 51 = (17)(3).

# Tag: 51

## Another View of the Regular Henkaipentacontagon

This is simply the inverted-color version of the second image in the last post on this blog, where I explain how to construct this 51-sided polygon using compass and straightedge, starting with the heptadecagon, which has 17 sides.

## Constructing the Henkaipentacontagon: A Regular Polygon with 51 Sides

After completing the heptadecagon construction shown in the last post on this blog, I wondered if I could pull off a similar trick to the one mentioned there of combining the pentagon and triangle constructions to construct a regular pentadecagon — but using the heptadecagon and triangle, instead, to construct a regular polygon with (17)(3) = 51 sides, known as the henkaipentacontagon.

The answer: yes, I was able to, but certainly not in the simplest way possible, for I ended up having to go to 204, first, to get to 51. First, I extended two adjacent radii of the heptadecagon in the upper left as rays, just to give me room to work. Next, I placed point B_{1 }on the lower of those two rays, to be used as the center of the large circle in which to construct my 51-sided regular polygon. I then constructed a line through B_{1} which was parallel to the upper of these two rays, thus duplicating the ~21.17647º central angle of the heptadecagon, but in the center of my new, larger circle. Next, I constructed the yellow equilateral triangle with this heptadecagon-central-angle inside it, in such a way that the lower half of the yellow triangle would be a 30-60-90 triangle, ΔA_{1}B_{1}C_{1,} with the ~21.17647° angle inside, and sharing a ray with, this triangle’s 30° angle. By subtraction, that made a ~8.8235° angle, with its vertex at the center of the largest circle shown.

Next, I divided ~8.8235 into 360 . . . and, to my dismay, didn’t get a whole number as an answer, but 40.8, instead. I then noticed that one can multiple 40.8 by five, and obtain 204 as the product. Armed with this knowledge, I used my ~8.8235° angle, and 204 circles of equal radius, to locate 204 points, evenly-spaced, around the large circle.

51 is one-fourth of 204, so I connected every fourth point around the large circle with heavy blue segments, and made those 51 points (one fourth of the total) blue, as well. These blue points and segments are the sides and vertices of the regular henkaipentacontagon, shown inscribed, above, inside the largest circle in the diagram.

Since this polygon looks a *lot *like a circle, I then rendered a lot of things from the diagram above invisible, in order to produce this second image: the henkaipentacontagon alone, with different colors for its vertices and sides, all radii added, and three alternating colors for the 51 triangles each formed by two adjacent radii and a side.