JEE Main & Advanced Sample Paper JEE Main - Mock Test - 19

  • 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 :

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


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