JEE Mains Indefinite Integration 2

Calculus Level 3

( log x 1 1 + ( log x ) 2 ) 2 d x = ? \large \int \left(\frac{\log x-1}{1+(\log x)^2}\right)^2 dx = ?

Notation: C C in the answer options denote the constant of integration .

This problem is a part of My Picks for JEE Mains 1
x ( log x ) 2 + 1 + C \dfrac{x}{(\log x)^2+1}+C x e x x 2 + 1 + C \dfrac{xe^x}{x^2+1}+C x x 2 + 1 + C \dfrac{x}{x^2+1}+C log x ( log x ) 2 + 1 + C \dfrac{\log x}{(\log x)^2+1}+C

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2 solutions

Tapas Mazumdar
Jul 12, 2018

Let I = ( ln x 1 1 + ( ln x ) 2 ) 2 d x \displaystyle I = \int {\left( \dfrac{\ln x -1}{1+ (\ln x)^2} \right)}^2 \,dx

Substituting ln x = u x = e u d x = e u d u \ln x = u \implies x = e^u \implies dx = e^u du , we get

I = ( u 1 1 + u 2 ) 2 e u d u = u 2 + 1 2 u ( 1 + u 2 ) 2 e u d u = ( 1 1 + u 2 + ( 2 u ) ( 1 + u 2 ) 2 ) e u d u = e u 1 + u 2 + C Using ( f ( x ) + f ( x ) ) e x d x = e x f ( x ) + C = x ( ln x ) 2 + 1 + C \begin{aligned} \displaystyle I &= \int {\left( \dfrac{u -1}{1+ u^2} \right)}^2 e^u \,du \\ \displaystyle &= \int \dfrac{u^2+1-2u}{(1+u^2)^2} e^u \,du \\ \displaystyle &= \int \left( \color{#20A900}{\dfrac{1}{1+u^2}} + \color{#D61F06}{\dfrac{(-2u)}{(1+u^2)^2}} \right) e^u \,du \\ \displaystyle &= \dfrac{e^u}{1+u^2} + C & \small{\text{Using } \displaystyle \int ( {\color{#20A900}{ f(x)}} +{ \color{#D61F06}{f'(x)}}) e^x \,dx = e^x f(x) + C} \\ &= \dfrac{x}{(\ln x)^2 +1} + C \\ \end{aligned}

Yash Ghaghada
Mar 2, 2018

Easily visible from the options

Because differentiation of (lnx)^2 gives 1\x so x should be there in numerator

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