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I know $x^3-3 x+1=0$ has three roots that can be expressed in trigonometric form: $\{2\sin(10^\circ),\,-2\cos(20^\circ),\,2\cos(40^\circ)\}$.

How can I get this result with Mathematica?

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2 Answers 2

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You can apply an identity directly:

ComplexExpand@
  Solve[x^3 - 3 x + 1 == 0, x] /. (A_: 1) Cos[t_] + (B_: 1) Sin[t_] :>
   Sqrt[A^2 + B^2] Cos[t - ArcTan[A, B]]

(* {{x -> 2 Cos[(2 π)/9]}, {x -> 2 Sin[π/18]}, {x -> -2 Cos[π/9]}} *)
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(ComplexExpand@Solve[x^3 - 3 x + 1 == 0, x] /. Pi/9 -> t // TrigFactor) /. t -> Pi/9

(*{{x -> 2 Cos[(2 π)/9]}, {x -> 2 Sin[π/18]}, {x -> -2 Cos[π/9]}}*)
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