Gauss theorem states that an is constructible ,Originally Posted by suedenation
where and are distinct Fermat primes (note can be ).
If an angle of degrees were constructible so would a sided
The first three Fermat primes are , clearly and
do not divide , so for the to be constructible
would have to be a power of , or (as it is ) would
have to be a power of . They are not so the is not
constructible and so an angle of 20 degrees is not constructible.