JEE Main & Advanced Mathematics Limits, Continuity and Differentiability Question Bank Self Evaluation Test - Continuity and Differentiability

  • question_answer
    If \[{{(x-a)}^{2}}+{{(y-b)}^{2}}={{c}^{2}}\], for some c > 0, then\[\frac{{{\left[ 1+{{\left( \frac{dy}{dx} \right)}^{2}} \right]}^{\frac{3}{2}}}}{\frac{{{d}^{2}}y}{d{{x}^{2}}}}\] is

    A) Is a constant dependent on a

    B) Is a constant dependent on b

    C) Is a constant independent of a and b

    D) 0

    Correct Answer: C

    Solution :

    [c] Given relation is \[{{(x-a)}^{2}}+{{(y-b)}^{2}}={{c}^{2}},c>0\]
    Let \[x-a=c\,\,\cos \theta \] and \[y-b=c\sin \theta \]. Therefore,
    \[\frac{dx}{d\theta }=-c\sin \theta \] and \[\frac{dy}{d\theta }=c\cos \theta \]
    \[\therefore \frac{dy}{dx}=-\cot \theta \]
    Differentiating both sides with respect to \[\theta \], we get
    \[\frac{d}{d\theta }\left( \frac{dy}{dx} \right)=\frac{d}{d\theta }(-cot\theta )\]
    or  \[\frac{d}{dx}\left( \frac{dy}{dx} \right)\frac{dx}{d\theta }=\cos e{{c}^{2}}\theta \]
    or \[\frac{{{d}^{2}}y}{d{{x}^{2}}}(-c\,sin\theta )=\cos e{{c}^{2}}\theta \]
    or \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=\frac{\cos e{{c}^{2}}\theta }{c}\]
    \[\therefore \frac{{{\left[ 1+{{\left( \frac{dy}{dx} \right)}^{2}} \right]}^{\frac{3}{2}}}}{\frac{{{d}^{2}}y}{d{{x}^{2}}}}=\frac{c{{\left[ 1+{{\cot }^{2}}\theta  \right]}^{\frac{3}{2}}}}{-\cos e{{c}^{3}}\theta }=\frac{c{{(cose{{c}^{2}}\theta )}^{\frac{3}{2}}}}{-\cos e{{c}^{3}}\theta }\]
    \[=-c.\]
    Which is constant and is independent of a and b.


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