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

  • question_answer
    \[y=a{{e}^{mx}}+b{{e}^{-mx}}\] satisfies which of the following differential equations    [Karnataka CET 2002]

    A)                 \[\frac{dy}{dx}-my=0\]      

    B)                 \[\frac{dy}{dx}+my=0\]

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

    D)                 \[\frac{{{d}^{2}}y}{d{{x}^{2}}}-{{m}^{2}}y=0\]

    Correct Answer: D

    Solution :

                       \[y=a{{e}^{mx}}+b{{e}^{-mx}}\].         Differentiating, we get \[\frac{dy}{dx}=ma{{e}^{mx}}-mb{{e}^{-mx}}\].                    Differentiating again, we get \[\frac{{{d}^{2}}y}{d{{x}^{2}}}={{m}^{2}}a{{e}^{mx}}+{{m}^{2}}b{{e}^{-mx}}\]                                 \[={{m}^{2}}(a{{e}^{mx}}+b{{e}^{-mx}})={{m}^{2}}y\] or \[\frac{{{d}^{2}}y}{d{{x}^{2}}}-{{m}^{2}}y=0\].


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