KVPY Sample Paper KVPY Stream-SX Model Paper-20

  • question_answer
    The equation of a tangent to the parabola, which makes an angle \[\theta \] with the positive direction of x-axis, is:

    A) \[y=x\,\tan \,\theta +2\,\,\cot \,\theta \]

    B) \[y=x\,\tan \,\theta -2\,\cot \theta \]

    C)  \[x=y\cot \,\theta +2\tan \theta \]

    D) \[x=y\,\cot \theta -2\,\tan \theta \]

    Correct Answer: C

    Solution :

    \[{{x}^{2}}=8y\]
    \[\Rightarrow \]   \[\frac{dy}{dx}=\frac{x}{4}=\tan \theta \]
    \[\therefore \]      \[{{x}_{1}}=4\tan \theta \]
    \[{{y}_{1}}=2\,\,{{\tan }^{2}}\theta \]
    Equation of tangent:
    \[y-2\,\,{{\tan }^{2}}\theta =\tan \,\theta (x-4tan\theta )\]
    \[x=y\cot \theta +2\,\tan \theta .\]


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