JEE Main & Advanced Sample Paper JEE Main - Mock Test - 33

  • question_answer
    At what point on the curve \[{{x}^{3}}-\,\,8{{a}^{2}}y=0\], the slope of the normal is -

    A) (a, a)    

    B)        (2a, - a)

    C) (2a, a)  

    D)        None of these

    Correct Answer: C

    Solution :

    \[{{x}^{3}}-8{{a}^{2}}y=0\Rightarrow \,\,3{{x}^{2}}-8{{a}^{2}}\cdot \frac{dy}{dx}=0\] \[\Rightarrow \,\,\,3{{x}^{3}}=8{{a}^{2}}\cdot \frac{dy}{dx}\Rightarrow \,\frac{dy}{dx}=\frac{3{{x}^{2}}}{8{{a}^{2}}}\] \[\therefore \,\,slope\,\,of\,\,the\,\,normal=-\frac{1}{\left( \frac{dy}{dx} \right)}=-\frac{1}{\frac{3{{x}^{2}}}{8{{a}^{2}}}}=-\frac{8{{a}^{2}}}{3{{x}^{2}}}\] \[\frac{-\,8{{a}^{2}}}{3{{x}^{2}}}=\frac{-2}{3}\Rightarrow \,\,(x,\,\,y)=(2a,\,\,a)\]


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