JEE Main & Advanced Sample Paper JEE Main Sample Paper-32

  • question_answer
    If \[\frac{dy}{dx}=\frac{{{y}^{3}}}{{{e}^{2x}}+{{y}^{2}}}\], and \[y\left( 0 \right)=1,\]then

    A) \[{{y}^{2}}=4{{e}^{2x}}-2{{e}^{2x}}\,\,\ell ny\]

    B) \[{{y}^{2}}={{e}^{2x}}+2{{e}^{2x}}\,\,\ell ny\]

    C) \[{{y}^{2}}={{e}^{2x}}-\frac{1}{2}{{e}^{2x}}\,\,\ell ny\]

    D) \[{{y}^{2}}={{e}^{2x}}+\frac{1}{2}{{e}^{2x}}\,\,\ell ny\]

    Correct Answer: A

    Solution :

    \[\frac{dx}{dy}=\frac{{{e}^{2x}}+{{y}^{2}}}{{{y}^{3}}}\Rightarrow {{e}^{-2x}}\frac{dx}{dy}=\frac{1}{{{y}^{3}}}+\frac{1}{y}{{e}^{-2x}}\]                 Put \[-{{e}^{-2x}}=t\Rightarrow \frac{dt}{dy}+\frac{2t}{y}=\frac{2}{{{y}^{3}}}\]                                 (Linear differential equation in t) \[\therefore \,\,\,\,t.{{y}^{2}}=\int{\frac{2}{{{y}^{3}}}}.{{y}^{2}}dy+C\] \[\Rightarrow \,-{{e}^{-2x}}.{{y}^{2}}=2\ell n\,y+C.\]Now \[y(0)=1\] \[\Rightarrow \,C=-1\] \[\therefore \,\,-{{e}^{-2x}}.{{y}^{2}}=2\ell n\,\,y-1\] \[\Rightarrow \,\,{{y}^{2}}={{e}^{2x}}-2{{e}^{2x}}\ell n\,\,y\]


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