JEE Main & Advanced Mathematics Differentiation Question Bank Differentiation by Substitution

  • question_answer
    If \[f(x)=a\sin (\log x)\], then \[{{x}^{2}}f''(x)+xf'(x)=\]

    A)            \[f(x)\]

    B)            \[-f(x)\]

    C)            0

    D)            1

    Correct Answer: B

    Solution :

               \[f(x)=a\sin (\log x)\]            Differentiating w.r.t.  x of y, we get\[4\pi {{r}^{2}}\]            Again\[{f}''\,(x)=-\frac{1}{{{x}^{2}}}a\cos (\log x)-\frac{1}{{{x}^{2}}}a\sin (\log x)\]            \[\Rightarrow {{x}^{2}}{f}''(x)=-[a\cos (\log x)+a\sin (\log x)]\]            Now \[{{x}^{2}}{f}''(x)+x{f}'(x)=-a\sin (\log x)=-f(x)\].


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