JEE Main & Advanced JEE Main Paper (Held On 8 April 2017)

  • question_answer
    If\[y={{\left[ x+\sqrt{{{x}^{2}}-1} \right]}^{15}}+y{{\left[ x-\sqrt{{{x}^{2}}-1} \right]}^{15}},\]then\[({{x}^{2}}-1)\frac{{{d}^{2}}y}{d{{x}^{2}}}+x\frac{dy}{dx}\]is equal to: [JEE Online 08-04-2017]

    A) \[224{{y}^{2}}\]               

    B) \[125y\]

    C) \[225y\]                              

    D) \[225{{y}^{2}}\]

    Correct Answer: C

    Solution :

                    \[y=\left\{ x+{{\sqrt{{{x}^{2}}-1}}^{15}} \right\}+{{\left\{ x-\sqrt{{{x}^{2}}-1} \right\}}^{15}}\] \[\frac{dy}{dx}=15{{\left( x+\sqrt{{{x}^{2}}-1} \right)}^{14}}15{{\left( x-\sqrt{{{x}^{2}}-1} \right)}^{14}}\left( 1-\frac{x}{\sqrt{{{x}^{2}}-1}} \right)\]                 \[\frac{dy}{dx}=\frac{15}{\sqrt{{{x}^{2}}-1}}.y\]                                ?(i)                 \[\sqrt{{{x}^{2}}-1}.\frac{dy}{dx}=15y\]                 \[\frac{x}{\sqrt{{{x}^{2}}-1}}.\frac{dy}{dx}+\sqrt{{{x}^{2}}-1}\frac{{{d}^{2}}y}{d{{x}^{2}}}=15\frac{dy}{dx}\] \[x\frac{dy}{dx}+\left( {{x}^{2}}-1 \right)\frac{{{d}^{2}}y}{d{{x}^{2}}}=15\sqrt{{{x}^{2}}-1}.\frac{15}{\sqrt{{{x}^{2}}-1}}.y=225y\]


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