trigonometry

Question: If $\cos \theta =\frac{1}{2}\left( a+\frac{1}{a} \right),$then the value of $\cos 3\theta $is

1) $\frac{1}{8}\left( {{a}^{3}}+\frac{1}{{{a}^{3}}} \right)$

2) $\frac{3}{2}\left( a+\frac{1}{a} \right)$

3) $\frac{1}{2}\left( {{a}^{3}}+\frac{1}{{{a}^{3}}} \right)$

4) $\frac{1}{3}\left( {{a}^{3}}+\frac{1}{{{a}^{3}}} \right)$

Solution:

Explanation: No Solution

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