JEE Main & Advanced JEE Main Paper Phase-I Held on 07-1-2020 Morning

  • question_answer
    Let \[{{x}^{k}}+{{y}^{k}}={{a}^{k}},\text{ (}a,\text{ }k>0)\] and \[\frac{dy}{dx}+{{\left( \frac{y}{x} \right)}^{\frac{1}{3}}}=0\], then k is:                [JEE MAIN Held on 07-01-2020 Morning]

    A) \[\frac{3}{2}\]              

    B) \[\frac{1}{3}\]

    C) \[\frac{4}{3}\]              

    D) \[\frac{2}{3}\]

    Correct Answer: D

    Solution :

    [d] Given \[{{x}^{k}}+{{y}^{k}}={{a}^{k}}\] Differentiating, \[k\cdot {{x}^{k-1}}+k\cdot {{y}^{k-1}}\frac{dy}{dx}=0\] \[\therefore \frac{dy}{dx}=-{{\left( \frac{x}{y} \right)}^{k-1}}\] \[\left( \frac{dy}{dx} \right)+{{\left( \frac{y}{x} \right)}^{1-k}}=0\] \[\therefore 1-k=\frac{1}{3}\] \[\Rightarrow k=\frac{2}{3}\]


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