# Would a $\infty$ sided regular polygon describe a circle?

Help us settle this argument. Consider a circle. What would best describe a circle?

A zero sided regular polygon? The other person thinks this because it cannot be infinite, because the shape is enclosed. There is no end, and there has to be a limit on size.

I disagree with this because the size can be made infinitely small so that it can have infinite sides. Also, zero sides= nothing

An infinite sided polygon I think this because a tangent has a angle of 180 where it meets an edge, and for a $$n$$ sided polygon, the interior angle converges to 180 when $$n$$ goes towards $$\infty$$

The other person disagrees with this because apparently, size cannot be unlimited.

# Could you say who is right?

• A circle is defined by its radius, so a fairer comparison would be between the apothem of a polygon and the radius of a circle, and not one using interior angles, since a circle has none. Apr 30 '17 at 21:31
• The first thing one should ask in such discussions is: "What is a side?". Apr 30 '17 at 21:38

A circle is not a polygon with infinite sides, and it's not a polygon with zero sides. It's not a polygon at all. This boils down to an argument about terminology - what does it mean for something to have an infinite number of sides? This would have to be precisely defined before the question makes sense. However, what's clear is that as the number of sides of a regular polygon tends to infinity, the shape approaches a circle (although there are a number of ways to interpret this statement in precise mathematical terms.)

• It says describe, not represent, but accepting in 30 min approx.... Apr 30 '17 at 21:41
• @simplest_mathematics? What is the difference between describe and represent? Well, clearly, this is informal question, and this answer fairly addresses the issue that the question has too many undefined terms to be precisely answered. Apr 30 '17 at 21:45

If you consider the sequence of regular polygons with $n$-sides and inscribe them in a circle. You can see quite clearly that, under the limit as $n\rightarrow\infty$, the polygons are approaching the circle.

• You can even see every regular polygon has a constant for its area just like a circle has pi. Beginning from an equilateral triangle, c=3sqrt(3)/2, and the number approaches pi as n goes to infinity
– Jack
Apr 30 '17 at 21:52

One way to settle this would be to use limits. For example, the formula for the area of a regular polygon of $n$ sides given its apothem $a$ is $$a^2n\tan\frac{180}{n}$$ The limit of this as $n$ approaches infinity is given by $\pi a^2$, so I would say that area-wise, the analogy seems to make sense. You could use a different limit to draw a comparison between the circumference formula.

In the infinite side case, each side is a point, right? Since in a regular polygon all the vertices are equidistant from a central point, then we have an infinite collection of points which are vertices, thus an infinite collection of points equidistant from a central point, which is exactly a circle I would think.