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

  • question_answer
    If \[\cos \theta -\sin \theta =\sqrt{2}\sin \theta ,\]then \[\cos \theta +\sin \theta \]is equal to  [WB JEE 1988]

    A) \[\sqrt{2}\cos \theta \]

    B) \[\sqrt{2}\sin \theta \]

    C) \[2\cos \theta \]

    D) \[-\sqrt{2}\cos \theta \]

    Correct Answer: A

    Solution :

    We have \[\cos \theta -\sin \theta =\sqrt{2}\,\sin \theta \] \[\Rightarrow \,\cos \theta =(\sqrt{2}+1)\,\sin \theta \,\Rightarrow \,(\sqrt{2}-1)\cos \theta =\sin \theta \] \[\Rightarrow \,\sqrt{2}\,\cos \theta -\cos \theta =\sin \theta \Rightarrow \,\sin \theta +\cos \theta =\sqrt{2}\,\cos \theta .\]


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