$\cos 15^\circ = $

  • A
    $\sqrt{\frac{1 + \cos 30^\circ}{2}}$
  • B
    $\sqrt{\frac{1 - \cos 30^\circ}{2}}$
  • C
    $\pm \sqrt{\frac{1 + \cos 30^\circ}{2}}$
  • D
    $\pm \sqrt{\frac{1 - \cos 30^\circ}{2}}$

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