यदि $\cos \theta = \frac{1}{2}\left( a + \frac{1}{a} \right)$ है,तो $\cos 3\theta$ का मान क्या है?

  • A
    $\frac{1}{8}\left( a^3 + \frac{1}{a^3} \right)$
  • B
    $\frac{3}{2}\left( a + \frac{1}{a} \right)$
  • C
    $\frac{1}{2}\left( a^3 + \frac{1}{a^3} \right)$
  • D
    $\frac{1}{3}\left( a^3 + \frac{1}{a^3} \right)$

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