JEE Main & Advanced Mathematics Trigonometric Identities Question Bank Fundamental trigonometrical ratios and functions, Trigonometrical ratio of allied angles

  • question_answer
    The value of \[\cos y\cos \left( \frac{\pi }{2}-x \right)-\cos \left( \frac{\pi }{2}-y \right)\cos x\] \[+\sin y\cos \left( \frac{\pi }{2}-x \right)+\cos x\sin \left( \frac{\pi }{2}-y \right)\] is zero, if

    A) \[x=0\]

    B) \[y=0\]

    C) \[x=y\]

    D) \[x=n\pi -\frac{\pi }{4}+y,\,\,(n\in I)\]

    Correct Answer: D

    Solution :

    The expression is equal to \[\sin (x-y)+\cos (x-y)=\sqrt{2}\left\{ \sin \left( \frac{\pi }{4}+x-y \right) \right\}\], which is zero, if \[\sin \left( \frac{\pi }{4}+x-y \right)=0\] i.e., \[\frac{\pi }{4}+x-y=n\pi (n\in I)\Rightarrow x=n\pi -\frac{\pi }{4}+y\].


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