draw a perfect Icosikai-henagon

Icosikai-henagon(21 sides regular polygon) is also a nonconstructive polygon. But with some simple steps, you can draw a perfect Icosikai-henagon with little error of 0.0023°。

The method is to find out 2 times of the Icosikai-henagon agnel.

The detail steps are on the picture.

I am not show the rest steps as it is very easy.(use a compass along the circle two times you will get it.)