$\cos 15^\circ = $
$\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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