integrate (ln(2018x+1)/(1+x)^4 dx from 0 to infinte

Calculus Level 3

0 ln ( 2018 x + 1 ) ( 1 + x ) 4 d x = ? \large \int_0^\infty \dfrac{\ln(2018x+ 1)}{(1+x)^4} \, dx = \, ?


The answer is 2.04.

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1 solution

First, we do integration by parts

ln ( 1 + 2018 x ) ( 1 + x ) 4 d x = ln ( 1 + 2018 x ) 3 ( 1 + x ) 3 + 1 3 2018 ( 1 + 2018 x ) ( 1 + x ) 4 d x \displaystyle \int \dfrac{\ln(1+2018x)}{(1+x)^{4}} dx = -\dfrac{\ln(1+2018x)}{3(1+x)^{3}}+\dfrac{1}{3} \int \dfrac{2018}{(1+2018x)(1+x)^{4}} dx

The first term is zero when applying the limit of integration. The second fraction can be written as(using partial fraction technique)

( 2018 3 2017 ) ( 201 8 3 201 7 2 1 1 + 2018 x 201 8 2 201 7 2 1 1 + x 2018 2017 1 ( 1 + x ) 2 1 ( 1 + x ) 3 ) d x \displaystyle \int \left ( \dfrac{2018}{3 \cdot 2017} \right ) \left ( \dfrac{2018^{3}}{2017^{2}}\dfrac{1}{1+2018x}- \dfrac{2018^{2}}{2017^{2}}\dfrac{1}{1+x}-\dfrac{2018}{2017}\dfrac{1}{(1+x)^{2}}-\dfrac{1}{(1+x)^{3}}\right )dx

The exact form is then

2018 3 2017 ( ( 2018 2017 ) 2 ln ( 2018 ) 2018 2017 1 2 ) 2.039982229 \displaystyle \dfrac{2018}{3 \cdot 2017} \left ( \left ( \dfrac{2018}{2017} \right )^{2} \ln(2018)-\dfrac{2018}{2017}-\dfrac{1}{2} \right ) \approx \boxed{2.039982229}

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