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3.Trigonometrical Ratios, Functions and Identities
easy
$\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}} $
Solution
(a) $\cos {15^o} = \sqrt {\frac{{1 + \cos (2 \times {{15}^o})}}{2}} $
$= \sqrt {\frac{{1 + \cos {{30}^o}}}{2}} $ .$\left( {\,\,\,\,\because \cos {{15}^o} > 0} \right)$
Standard 11
Mathematics
