KVPY Sample Paper KVPY Stream-SX Model Paper-11

  • question_answer
    If \[x=3\text{ }tan\text{ }t\] and \[y=3\text{ }\sec t\]  then the value of \[\frac{{{d}^{2}}y}{d{{x}^{2}}}\] at  \[t=\frac{\pi }{4},\] is:

    A) \[\frac{3}{2\sqrt{2}}\]

    B) \[\frac{1}{2\sqrt{2}}\]

    C) \[\frac{1}{6}\]                          

    D) \[\frac{1}{6\sqrt{2}}\]

    Correct Answer: D

    Solution :

    \[\frac{dx}{dt}=3{{\sec }^{2}}t\]
    \[\frac{dx}{dt}=3\sec t\tan t\]
    \[\frac{dy}{dx}=\frac{\tan t}{\sec t}=\sin t\]
    \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=\cos t\frac{dt}{dx}\]\[=\frac{\cos t}{3{{\sec }^{2}}t}=\frac{{{\cos }^{3}}t}{3}\]\[=\,\frac{1}{3.2\sqrt{2}}=\frac{1}{6\sqrt{2}}.\]


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