JEE Main & Advanced Mathematics Differential Equations Question Bank Variable Separable type differential equations

  • question_answer
    The solution of \[(\text{cosec}\,x\log y)dy+({{x}^{2}}y)dx=0\] is [AISSE 1986]

    A)                 \[\frac{\log y}{2}+(2-{{x}^{2}})\cos x+2\sin x=c\]

    B)                 \[{{\left( \frac{\log y}{2} \right)}^{2}}+(2-{{x}^{2}})\cos x+2x\sin x=c\]

    C)                 \[{{\frac{(\log y)}{2}}^{2}}+(2-{{x}^{2}})\cos x+2x\sin x=c\]

    D)                 None of these

    Correct Answer: C

    Solution :

                       \[(\text{cosec }x\log y)dy+({{x}^{2}}y)dx=0\]Þ \[\frac{1}{y}\log ydy=-{{x}^{2}}\sin xdx\]                    On integrating both sides, we get                    \[\frac{{{(\log y)}^{2}}}{2}+[{{x}^{2}}(-\cos x)+\int_{{}}^{{}}{2x\cos xdx}]=c\]                    Þ \[\frac{{{(\log y)}^{2}}}{2}-{{x}^{2}}\cos x+2(x\sin x+\cos x)=c\]                                 Þ \[\frac{{{(\log y)}^{2}}}{2}+(2-{{x}^{2}})\cos x+2x\sin x=c\].


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