JEE Main & Advanced JEE Main Paper (Held on 12-4-2019 Morning)

  • question_answer
    Consider the differential equation,\[{{y}^{2}}dx+\left( x-\frac{1}{y} \right)dy=0.\]If value of y is 1 when \[x=1,\] the value of x for which \[y=2,\]is :                [JEE Main Held on 12-4-2019 Morning]

    A) \[\frac{1}{2}+\frac{1}{\sqrt{e}}\]               

    B) \[\frac{3}{2}-\sqrt{e}\]

    C) \[\frac{5}{2}+\frac{1}{\sqrt{e}}\]               

    D) \[\frac{3}{2}-\frac{1}{\sqrt{e}}\]

    Correct Answer: D

    Solution :

    \[{{y}^{2}}dx+xdy=\frac{dy}{y}\]           \[\frac{dx}{dy}+\frac{x}{{{y}^{2}}}=\frac{1}{{{y}^{3}}}\]           \[IF={{e}^{\int_{{}}^{{}}{\frac{1}{{{y}^{2}}}dy}}}={{e}^{-\frac{1}{y}}}\]           \[{{e}^{-\frac{1}{y}}}.x=\int_{{}}^{{}}{{{e}^{-\frac{1}{y}}}}.\frac{1}{{{y}^{3}}}dy+C\]           \[x{{e}^{-\frac{1}{y}}}={{e}^{-\frac{1}{y}}}+\frac{{{e}^{-\frac{1}{y}}}}{y}+C\]           \[C=-\frac{1}{e}\]           \[x=\frac{3}{2}-\frac{1}{\sqrt{e}}\]when\[y=2\]          


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