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) \[\pm \sqrt{\frac{1-\cos 30{}^\circ }{2}}\]
Correct Answer: A
Solution :
\[\cos {{15}^{o}}=\sqrt{\frac{1+\cos (2\times {{15}^{o}})}{2}}=\sqrt{\frac{1+\cos {{30}^{o}}}{2}}\] \[\left( \because \,\,\,\,\cos {{15}^{o}}>0 \right)\].
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