JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    If \[y=A\cos nx+B\sin nx,\] then \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=\] [Karnataka CET 1996]

    A)            \[{{n}^{2}}y\]                       

    B)            \[-y\]

    C)            \[-{{n}^{2}}y\]

    D)            None of these

    Correct Answer: C

    Solution :

               \[y=A\cos (nx)+B\sin (nx)\]            \[\therefore dy/dx=-nA\sin (nx)+nB\cos (nx)\]            Again \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=-{{n}^{2}}A\cos (nx)-{{n}^{2}}B\sin (nx)\]                         \[=-{{n}^{2}}[A\cos (nx)+B\sin (nx)]\]            Þ  \[\frac{{{d}^{2}}y}{d{{x}^{2}}}=-{{n}^{2}}y\].


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