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

  • question_answer
    The solution of the equation \[\sqrt{a+x}\frac{dy}{dx}+x=0\] is [DSSE 1988]

    A)                 \[3y+2\sqrt{a+x}.(x-2a)=3c\]

    B)                 \[3y+2\sqrt{x+a}.(x+2a)=3c\]

    C)                 \[3y+\sqrt{x+a}.(x+2a)=3c\]           

    D)                 None of these

    Correct Answer: A

    Solution :

                       \[\sqrt{a+x}\frac{dy}{dx}+x=0\] Þ \[\int_{{}}^{{}}{dy}=-\int_{{}}^{{}}{\frac{x}{\sqrt{a+x}}dx}\]                    Þ \[y=-\int_{{}}^{{}}{\sqrt{a+x}}dx+\int_{{}}^{{}}{\frac{a}{\sqrt{a+x}}}dx\]            \[\left\{ \because \int_{{}}^{{}}{\frac{x}{\sqrt{a+x}}}dx=\int_{{}}^{{}}{\frac{x+a-a}{\sqrt{a+x}}}dx \right\}\]                    Þ \[y=-\frac{2}{3}{{(a+x)}^{3/2}}+2a\sqrt{a+x}+c\]                    Þ \[3y=-\sqrt{a+x}(2(a+x)-6a)+3c\]                    Þ \[3y=-2\sqrt{a+x}(x-2a)+3c\]                                 Þ \[3y+2\sqrt{a+x}(x-2a)=3c\].


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