Gauss theorem states that an is constructible ,Originally Posted bysuedenation

where and are distinct Fermat primes (note can be ).

If an angle of degrees were constructible so would a sided

polygon.

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.

RonL