JEE Main & Advanced Mathematics Differential Equations Question Bank Mock Test - Differential Equations

  • question_answer
    The differential equation of all parabolas whose axis are parallel to the y-axis is

    A) \[\frac{{{d}^{3}}y}{d{{x}^{3}}}=0\]

    B) \[\frac{{{d}^{2}}x}{d{{y}^{2}}}=C\]

    C) \[\frac{{{d}^{3}}y}{d{{x}^{3}}}+\frac{{{d}^{2}}x}{d{{y}^{2}}}=0\]

    D) \[\frac{{{d}^{2}}y}{d{{x}^{2}}}+2\frac{dy}{dx}=C\]

    Correct Answer: A

    Solution :

    [a] The equation of a member of the family of parabolas having axis parallel to y-axis is \[y=A{{x}^{2}}+Bx+C\]                    ...(1) Where A, B, and C are arbitrary constants. Differentiating equation (1) w.r.t. x, we get \[\frac{dy}{dx}=2Ax+B\]                       ...(2) Which on again differentiating w.r.t. x gives \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=2A\]                                 ...(3) Differentiating (3) w.r.t. x, we get \[\frac{{{d}^{3}}y}{d{{x}^{3}}}=0\]


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