$\int\left\{\frac{(\log x-1)}{\left(1+(\log x)^2\right.}\right\}^2 d x$ is equal to

$\int\left\{\frac{(\log x-1)}{\left(1+(\log x)^2\right.}\right\}^2 d x$ is equal to
  1. $\frac{\log x}{(\log x)^2+1}+C$
  2. $\frac{x}{x^2+1}+C$
  3. $\frac{x e^x}{1+x^2}+C$
  4. $\frac{x}{(\log x)^2+1}+C$

Solution

$ \begin{aligned} & \int \frac{(\log x-1)^2}{\left(1+(\log x)^2\right)^2} d x \\ & =\int\left[\frac{1}{\left(1+(\log x)^2\right)}-\frac{2 \log x}{\left(1+(\log x)^2\right)^2}\right] d x \\ & =\int\left[\frac{e^t}{1+t^2}-\frac{2 t e^t}{\left(1+t^2\right)^2}\right] d t \text { put } \log x=t \Rightarrow d x=e^t d t \\ & \int e^t\left[\frac{1}{1+t^2}-\frac{2 t}{\left(1+t^2\right)^2}\right] d t \\ & =\frac{e^t}{1+t^2}+c=\frac{x}{1+(\log x)^2}+c \end{aligned} $

Asked in: JEE Main 2005

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