The Math Forum

Ask Dr. Math - Questions and Answers from our Archives
Associated Topics || Dr. Math Home || Search Dr. Math

Impossibility of Constructing a Regular Nine-Sided Polygon

Date: 04/07/98 at 23:01:23
From: fei su
Subject: method to draw a regular 9 sides polygon

Dear Dr. Math, 

My brother was given an easy method to draw a regular 9 sided polygon. 
His tools were only a compass and a ruler that has no units marked 
on it. Naturally, the regular 9 sided polygon is an approximate one 
judged by common sense. What he has been doing was proving it to be a 
real regular polygon, but he has not gotten the answer after spending 
more than a dozen years. I think that maybe there is no such answer. 
My question is: Is this drawing method useful? Is it worth it to find 
another easy method to draw a regular 9 sided polygon? Is there any 
value to spending time on this issue?

Date: 04/08/98 at 11:53:34
From: Doctor Wilkinson
Subject: Re: method to draw a regular 9 sides polygon

The problem is not one of practical importance, but it is certainly of 
great intellectual interest. Unfortunately for your brother, this 
problem is known to be impossible, so it would be better for him to 
work on something else.

-Doctor Wilkinson, The Math Forum
Check out our web site!   

Date: 04/10/98 at 15:02:10
From: Doctor Wilkinson
Subject: Re: method to draw a regular 9 sides polygon

Hi. I think I may have given too brief a reply to your question about 
constructing a regular nine-sided polygon. I certainly do not want to 
discourage your brother or you from working on hard problems!

First of all, when we say a construction problem with straightedge and
compass is impossible, you may wonder how we can possibly know that. 
You may think that there is some trick that just hasn't been tried 
yet. The way we know some problems are impossible is that we can 
figure out just what kinds of things can and can't be done with 
straightedge and compass, and we can describe all this in terms of 

What can you do with a straightedge and compass really? You can make 
circles with a radius you've already got, you can draw lines between 
points, and you can get new points as the intersections of lines and 
circles or of circles and circles.

Now the great French mathematician Rene Descartes discovered in the
sixteenth century a way to make equations corresponding to geometric
lines and curves. In particular, the equation of a straight line 
always has the form:

     ax + by = c

and the equation of a circle has the form:

     (x - a)^2 + (y - b)^2 = r^2

where a, b, c, and r are constants, and x and y are the coordinates of 
a point on the line or circle.

Finding the intersection of two lines, of a line and a circle, or of 
two circles thus is equivalent to solving a quadratic equation.

So any construction that is possible with straightedge and compass is
equivalent to an algebraic problem whose solution involves only 
addition, subtraction, multiplication, division, or extracting square 

Using these ideas, Gauss was able to show around 1801 that the only
regular polygons that could be constructed with straightedge and 
compass were those for which the number of sides was a product of a 
power of 2 and of distinct primes of the form 2^2^n + 1. For example,
a regular polygon with 30 sides can be constructed because 30 is 
2*3*5, and 3 is 2 + 1 or 2^2^0 + 1 and 5 is 2^2 + 1 or 2^2^1 + 1. But 
9 is 3 * 3, and while 3 is a prime of the right form, the fact that 
you have 2 factors of 3 means it won't work. Gauss was the first to 
construct a regular polygon with 17 sides, which you can see is 
possible because 17 is 2^2^2 + 1.

The primes of the form 2^2^n + 1 are known as Fermat primes, because
Pierre Fermat, another Frenchman and a contemporary of Descartes, 
thought that perhaps all number of that form were prime. But in fact 
the only known primes of that form correspond to n = 0, 1, 2, 3, and 
4. The next number of that form, 2^2^5 + 1, is not prime, so you 
couldn't construct a regular polygon with that number of sides even if 
you had time.

There are a bunch of hard unsolved problems relating to these numbers.  
It isn't known whether there are any more primes among them, or 
whether an infinite number of them are prime, or whether an infinite 
number are composite.

There are plenty of unsolved problems all through mathematics. Here's 
a webpage that will steer you to some of them:   

Best wishes, and I apologize if my earlier reply was too short and too

-Doctor Wilkinson, The Math Forum
Check out our web site!   
Associated Topics:
High School About Math
High School Constructions
High School Geometry
High School History/Biography
High School Triangles and Other Polygons

Search the Dr. Math Library:

Find items containing (put spaces between keywords):
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

Math Forum Home || Math Library || Quick Reference || Math Forum Search

Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.