JEE Main & Advanced JEE Main Paper (Held On 16 April 2018)

  • question_answer
    The differential equation representing the family of ellipse having foci either on the x-axis or on the y-axis, centre at the origin and passing through the point \[(0,3)\]is?

    A) \[xyy'+{{y}^{2}}-9=0\]     

    B)  \[x+y{{y}^{''}}=0\]

    C)  \[xyy''+x{{(y')}^{2}}-yy'=0\]

    D)  \[xyy'-{{y}^{2}}+9=0\]

    Correct Answer: C

    Solution :

                     \[\frac{{{x}^{2}}}{{{a}^{2}}}+\frac{{{y}^{2}}}{{{b}^{2}}}=1\]                 it passes through (0,3), so it will become                 \[\frac{{{x}^{2}}}{{{a}^{2}}}+\frac{{{y}^{2}}}{9}=1\] different w.r.t x, we get \[\frac{2x}{{{a}^{2}}}+\frac{2y}{9}\frac{dy}{dx}=0\] \[\frac{y}{x}\left( \frac{dy}{dx} \right)=\frac{-9}{{{a}^{2}}}\] diff w.r.t x, we get \[(\frac{y}{x}\frac{{{d}^{2}}y}{d{{x}^{2}}})+\frac{x\frac{dy}{dx}-y}{{{x}^{2}}}\frac{dy}{dx}=0\] \[xyy''+xy{{'}^{2}}-yy'=0\]


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