If $x + \frac{1}{x} = 2\,\cos \theta ,$ then ${x^3} + \frac{1}{{{x^3}}} = $
If $x + \frac{1}{x} = 2\,\cos \theta ,$ then ${x^3} + \frac{1}{{{x^3}}} = $
- A
$\cos \,\,3\theta $
- B
$2\,\cos \,3\theta $
- C
$\frac{1}{2}\cos \,3\theta $
- D
$\frac{1}{3}\cos \,3\theta $
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