"Detailed investigations on the famous old problem of the section of an angle into an odd number of equal parts, led Vieta to the discovery of a trigonometrical solution of Cardan's irreducible case in cubics. He applied the equation (2 cos 1⁄3 φ)3 - 3 (2 cos 1⁄3 cos φ) = 2 cos φ to the solution of x3 - 3 a2x = a2b, when a > ½ b, by placing x = 2 a cos 1⁄3 φ, and determining φ from b = 2a cos φ."