10th Class Mathematics Introduction to Trigonometry Question Bank MCQs - Introduction to Trigonometry

  • question_answer
    If \[\sec \theta +\tan \theta =m,\] then the value of \[{{\sec }^{4}}\theta -{{\tan }^{4}}\theta -2\sec \theta \,\tan \theta \] is:

    A) \[{{m}^{2}}\]

    B) \[\frac{1}{{{m}^{2}}}\]

    C) \[\frac{1}{m}\]

    D) None of these

    Correct Answer: B

    Solution :

    [b]  \[\sec \theta +\tan \theta =m\]
    \[\therefore \,\,\,\,\,\sec \theta -\tan \theta =\frac{1}{m}\]
    Now, \[{{\sec }^{4}}\theta -{{\tan }^{4}}\theta -2\sec \theta \cdot \tan \theta \]
    \[=({{\sec }^{2}}\theta -{{\tan }^{2}}\theta )({{\sec }^{2}}\theta +{{\tan }^{2}}\theta )-2\sec \theta \cdot \tan \theta \]\[=1\cdot ({{\sec }^{2}}\theta +{{\tan }^{2}}\theta )-2\sec \theta \cdot \tan \theta \]
    \[={{(\sec \theta -\tan \theta )}^{2}}\]\[[{{a}^{2}}+{{b}^{2}}-2ab={{(a-b)}^{2}}]\]
    \[=\frac{1}{{{m}^{2}}}\]


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