## Regular polygon equation

A while ago, a blog reader named BJ (he seems to prefer being called BJ. He? *checks email…* Yeah, he) emailed me with his answer to this question: Is there an equation to describe regular polygons?

I’m not clever enough to do much editing and explanation, so I’ll post his email (got his permission and clarified some points) here.

*Start email quote*

An example: A polygon equation can be approximated by a single continuous implicit equation. Suppose x*y=0.1 The concept is to think of this hyperbola as almost being the product of two lines intersecting at x=0 and y=0.

Construct a (3,4,5) right triangle like this: Begin with the multiplication (x – 1)(y + 1) = 0.1. This product is the first approximated vertex. Multiply a second time using (3y – 4x – 5). This creates two more curved vertices. The multiplication expression now has three factors on the LHS. The RHS remains 0.1. The final equation will form a figure with the approximated triangle being a central “island”. Below is an equation for a (3,4,5) right triangle that morphs from intersecting lines to the triangle and then on to the circle. I sandwiched the approximated triangle between two circles, and it becomes one circle in the limit as 0.1 -> 0.

((x + 0.5)^2 + (y – 1)^2 – 6.125) – ((x + 0.5)^2 + (y – 1)^2 – 6.125)(3y – 4x – 5)(x – 1)(y + 1) = 0.1

Notice that I have multiplied by a circle in the last term. I sandwiched the approximated triangle between two circles, and it becomes one circle in the limit as .1 -> 0. The last term is subtracted from the circle that in the first term. The multiplication expression goes to zero, and only the circle remains. This method is somewhat analogous to the method used in Euclidean geometry.

*End email quote*

He also sent a follow-up email:

*Start email quote*

Here is a quadratic form for the triangle [(0,0),(4,0),(4,3)].
(x – 4) y (y – 0.75x) = 0.01

Solve for y=f(x) on the range 0 <= x <= 4 y = (15x^2 - 60x + sqrt(225x^4 - 1800x^3 + 3600x^2 + 16x - 64)) / (40x - 160) y = (15x^2 - 60x - sqrt(225x^4 - 1800x^3 + 3600x^2 + 16x - 64)) / (40x - 160) Plot using two colors, one for each solution. *End email quote* You can add your thoughts on this in the comments.

### Story time

I don’t have much else to add to that, so I’ll just tell you a story instead.

Back when I was in university, there was a programming problem that’s to calculate the value of PI. There were 2 methods involved.

The first method used Monte Carlo simulation. Basically you have a circle with radius 1 unit. So the diameter is 2 units. And you have a square that just contains this circle, so the square is 2 units wide by 2 units high.

The area of the square is 2*2 = 4 units. The area of the circle is given by PI*r*r which is PI (because r is 1 unit). And the ratio of area of circle to area of square is PI/4.

Using Monte Carlo simulation, I randomly selected a point within the square. I made a note of whether the point was within the circle or without. And

[Number of points within circle]/[Total number of points] = PI/4

Since PI is the only “unknown”, there you have it. Solve for PI. The more points you use, the more accurate PI is.

The other method involves calculating the circumference of a circle. Suppose you have a square with a width of square root 2. This is chosen such that the diagonal length of the square is 2 units. This means the distance from the centre of the square to the corner point of the square (any of the 4 of them) is 1 unit.

Are you getting the idea yet?

The perimeter of this square is 4 * (square root of 2). Then we double the number of sides so we get an 8-sided polygon, an octagon. But still keeping the “from centre to outer-most point is 1 unit length” condition. Using some more maths, we get the length of one side of this octagon and multiply it by 8. That will be the perimeter.

As we keep doubling the number of sides, this polygon eventually approximates a circle. And so the perimeter of said (regular) polygon approaches the circumference of a circle, which is 2*PI*r, or just 2*PI (because r is 1 unit). Solve for PI.

### Exercise

To end this, I will leave it to you as an exercise to calculate the length of that regular polygon in the 2nd method. Start by understanding why one side of a square is square root of 2. Then continue with calculating the side length for an octagon.

Side note: The 2nd method terminates faster as an iterative process than the Monte Carlo simulation. However, it is also less “stable”. Hahaha… for extra credit, explain why it is less “stable”.

Extra side note: If people knew the formula for a circle is PI*r*r, wouldn’t they have known the value of PI already? This was why I found the Monte Carlo simulation method a little on the self-fulfilling side. You’re solving for something that you sort of know the value of. That’s weird, almost like cheating.

## Is there an equation to describe regular polygons?

So a blog reader, Michael Gmirkin, sent me an in-depth email about the possibility of the existence of a super equation that can describe any regular polygon. I wasn’t sure. For reference, you might want to check out these 2 blog posts about the equation for a square: question, answer.

I was going to just ask you here. Then I remembered there’s a Stack Overflow equivalent for maths. So I went there and asked the question. So if you know the answer, you can comment here, or go to the maths StackExchange site and earn yourself some points.

You can assume that the centre of the regular polygon in at the origin (0,0). Researching a little on the topic, I also learnt about the apothem, which is also the shortest distance from the centre to a polygon’s side. The “normal” radius is the distance from the centre to one of the regular polygon’s vertex.

If you trace 2 circles, one with the apothem and one with the radius, you get an inscribed circle and circumscribed circle respectively.