Constructing a Regular Heptagon Using Lill's Method
This Demonstration shows how to construct a regular heptagon using Lill's method for solving cubic equations.[more]
The points of a regular heptagon with vertices on a circle of radius 1 are given by . Since is a solution, if we divide the polynomial by , we get
If , then
leads to the cubic equation
It has solutions , , .
This follows from the trigonometric identity
Set and to get
which factors as
There are solutions when the points and coincide.[less]
 G. E. Martin, Geometric Constructions, New York: Springer, 1998 p. 45.