JEE Main & Advanced Sample Paper JEE Main Sample Paper-27

  • question_answer
    For the curve defined parametrically as \[y=3\sin \theta \,\cos \theta ,x={{e}^{\theta }}\sin \theta \] where \[\theta \in [0,\pi ]\], the tangent is parallel to x-axis when \[\theta \] is

    A)  \[\frac{3\pi }{4}\]                                 

    B)  \[\frac{\pi }{4}\]

    C)  \[\frac{\pi }{2}\]                                   

    D)  \[\frac{\pi }{6}\]

    Correct Answer: B

    Solution :

    \[x={{e}^{\theta }}.\,\sin \theta \] \[\Rightarrow \,\,\frac{dx}{d\theta }\,={{e}^{\theta }}(\sin \theta +\cos \theta )\] \[y=3\,\sin \theta .\,\cos \theta \] \[\Rightarrow \,\,\frac{dy}{d\theta }\,=3\,\cos 2\theta \] \[\therefore \,\,\frac{dy}{dx}=\frac{3\cos 2\theta }{{{e}^{\theta }}.\,(\sin \theta +\cos \theta )}\] \[\Rightarrow \frac{dy}{dx}\,=0,\,\] when \[\theta =\frac{\pi }{4}\]


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