JEE Main & Advanced Mathematics Differential Equations Question Bank Formation of differential equations

  • question_answer
    The differential equation of all parabolas whose axes are parallel to 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 :

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


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