BCECE Engineering BCECE Engineering Solved Paper-2008

  • question_answer
    The solution of\[(1+{{x}^{2}})\frac{dy}{dx}+2xy-4{{x}^{2}}=0\]is

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

    B)  \[3y(1+{{x}^{2}})=4{{x}^{3}}+c\]

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

    D)  \[3y(1+{{y}^{2}})=4{{x}^{3}}+c\]

    Correct Answer: B

    Solution :

    Given differential equation can be rewritten as \[\frac{dy}{dx}+\frac{2x}{1+{{x}^{2}}}y=\frac{4{{x}^{2}}}{1+{{x}^{2}}}\] It is a linear differential equation of the form                                 \[\frac{dy}{dx}+Py=Q\]                 \[\therefore \]  \[={{e}^{\int_{{}}^{{}}{\frac{2x}{1+{{x}^{2}}}dx}}}=(1+{{x}^{2}})\]                 \[\therefore \]  Solution is                 \[y.(1+{{x}^{2}})=\int_{{}}^{{}}{(1+{{x}^{2}}).\frac{4{{x}^{2}}}{1+{{x}^{2}}}}dx\]                 \[\Rightarrow \]\[y(1+{{x}^{2}})=\frac{4{{x}^{3}}}{3}+{{c}_{1}}\] \[\Rightarrow \]\[3y(1+{{x}^{2}})=4{{x}^{3}}+c\]


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