Is there an identity for the cos( 1/3 x ) ?

[swampwiz]

Main Question or Discussion Point

It seems that this term comes up in solving the cubic equation. While there is the identity for the half-angle, there doesn't seem to be one for third-angle.

 

[coolul007]

http://en.wikipedia.org/wiki/Trig_Identity

 

[JJacquelin]

Hi !

The identity involves complex terms :

 

[swampwiz]

OK, I understand the general form. I didn't think to use an integer fraction in lieu of an integer. (Actually, it appears that *any* number could be used there.)