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

  • question_answer
    The equation \[{{\sec }^{2}}\theta =\frac{4xy}{{{(x+y)}^{2}}}\]is only possible when [MP PET 1986; IIT 1996]

    A) \[x=y\]

    B) \[x<y\]

    C) \[x>y\]

    D) None of these

    Correct Answer: A

    Solution :

    Since \[{{\cos }^{2}}\theta \le 1\] \[{{\sec }^{2}}\theta =\frac{4xy}{{{(x+y)}^{2}}}\ge 1\Rightarrow 4xy\ge {{(x+y)}^{2}}\Rightarrow {{(x-y)}^{2}}\le 0\] It is possible only when\[x=y\], \[(\because x,\,y\in R)\].


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