JEE Main & Advanced Mathematics Trigonometric Identities Question Bank Trigonometrical ratios of multiple and sub multiple angles

  • question_answer
    \[\cos 15{}^\circ =\] [MP PET 1988; MNR 1978]

    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 :

    \[\cos {{15}^{o}}=\sqrt{\frac{1+\cos (2\times {{15}^{o}})}{2}}=\sqrt{\frac{1+\cos {{30}^{o}}}{2}}\]       \[\left( \because \,\,\,\,\cos {{15}^{o}}>0 \right)\].

You need to login to perform this action.
You will be redirected in 3 sec spinner