JEE Main & Advanced Mathematics Differential Equations Question Bank Self Evaluation Test - Differential Equations

  • question_answer
    The expression which is the general solution of the differential equation \[\frac{dy}{dx}+\frac{x}{1-{{x}^{2}}}y=x\sqrt{y}\] is

    A) \[\sqrt{y}+\frac{1}{3}(1-{{x}^{2}})=c{{(1-{{x}^{2}})}^{\frac{1}{4}}}\]

    B) \[y{{(1-{{x}^{2}})}^{\frac{1}{4}}}=c(1-{{x}^{2}})\]

    C) \[\sqrt{y}{{(1-{{x}^{2}})}^{\frac{1}{4}}}=\frac{1}{3}(1-{{x}^{2}})+c\]

    D) None of these

    Correct Answer: A

    Solution :

    [a] Divide the equation by \[\sqrt{y}\], we get
    \[{{y}^{-\frac{1}{2}}}\frac{dy}{dx}+\frac{x}{1-{{x}^{2}}}{{y}^{\frac{1}{2}}}=x\]
    Put \[{{y}^{\frac{1}{2}}}=z\Rightarrow \frac{1}{2}{{y}^{-\frac{1}{2}}}\frac{dy}{dx}=\frac{dz}{dx}\]
    \[2\frac{dz}{dx}+\frac{x}{1-{{x}^{2}}}z=x\Rightarrow \frac{dz}{dx}+\left( \frac{1}{2}\frac{x}{1-{{x}^{2}}} \right)z=\frac{x}{2}\]
    I. F. \[{{e}^{\int{\frac{1}{2}\left[ \frac{x}{1-{{x}^{2}}} \right]dx}}}={{e}^{-\frac{1}{4}\log (1-{{x}^{2}})}}={{(1-{{x}^{2}})}^{\frac{1}{4}}}\]
    The solution
    is \[z{{(1-{{x}^{2}})}^{-\frac{1}{4}}}=\int{\frac{x}{2}{{(1-x)}^{-\frac{1}{4}}}dx+c}\]
    \[\Rightarrow z{{(1-{{x}^{2}})}^{-\frac{1}{4}}}=\frac{1}{2}\int{{{(1-{{x}^{2}})}^{-\frac{1}{2}}}xdx+c}\]
    \[\Rightarrow \sqrt{y}{{(1-{{x}^{2}})}^{\frac{1}{4}}}=\frac{1}{2}\left( -\frac{1}{2} \right)\frac{{{(1-{{x}^{2}})}^{3/4}}}{3/4}+c\]
    \[\Rightarrow \,\,\,\sqrt{y}{{(1-{{x}^{2}})}^{\frac{1}{4}}}=\,\,-\frac{1}{3}{{\left( 1-{{x}^{2}} \right)}^{3/2}}+c\]
    \[\Rightarrow \sqrt{y}+\frac{1}{3}\left( 1-{{x}^{2}} \right)=c{{(1-{{x}^{2}})}^{\frac{1}{4}}}\]


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