We know that, cos(pi/3) = 1/2, and cos 2x = 2*(cos x)^2 - 1, giving :

cos(2*pi/3) = -1/2, and so:

cos 3h = -1/2

where h = 2*pi/9, 4*pi/9, 8*pi/9

From the relationship between cos 3h and cos h, we will obtain a cubic equation in terms of cosh.

*Edited on ***September 13, 2007, 11:36 am**