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Exercise 67Calculus › Single-variable calculus › Integration › Improper integralsL1Added 6 Nov 2026

A convergent improper integral of $\ln x/(1+x)^2$

#improper integral#integration by parts#logarithm

Statement

Compute
I=∫01ln⁡x(1+x)2 dx.I=\int_0^1\frac{\ln x}{(1+x)^2}\,dx.

Hint

The integrand has an integrable singularity at 00: ln⁡x/(1+x)2∼ln⁡x\ln x/(1+x)^2\sim\ln x, and ∫01ln⁡x dx\int_0^1\ln x\,dx converges. Integrate by parts on [X,1][X,1] with u=ln⁡xu=\ln x, dv=dx(1+x)2dv=\frac{dx}{(1+x)^2}, then let X→0+X\to0^+ using Xln⁡X→0X\ln X\to0.

Warning. One must take X>0X>0 and pass to the limit; do not write [−ln⁡x/(1+x)]01[-\ln x/(1+x)]_0^1 directly, since ln⁡x\ln x is not defined at 00.

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Solution

Integration by parts on [X,1][X,1]. For 0<X<10<X<1, set u(x)=ln⁡xu(x)=\ln x and v′(x)=1(1+x)2v'(x)=\frac{1}{(1+x)^2}, so u′(x)=1xu'(x)=\frac1x and v(x)=−11+xv(x)=-\frac{1}{1+x}. Then
I(X):=∫X1ln⁡x(1+x)2 dx=[−ln⁡x1+x]X1+∫X1dxx(1+x).I(X):=\int_X^1\frac{\ln x}{(1+x)^2}\,dx=\left[-\frac{\ln x}{1+x}\right]_X^1+\int_X^1\frac{dx}{x(1+x)}.

First term.
[−ln⁡x1+x]X1=−ln⁡12−(−ln⁡X1+X)=ln⁡X1+X.\left[-\frac{\ln x}{1+x}\right]_X^1=-\frac{\ln 1}{2}-\left(-\frac{\ln X}{1+X}\right)=\frac{\ln X}{1+X}.

Second term. Partial fractions: 1x(1+x)=1x−11+x\frac{1}{x(1+x)}=\frac1x-\frac1{1+x}. So
∫X1dxx(1+x)=[ln⁡x−ln⁡(1+x)]X1=−ln⁡2−(ln⁡X−ln⁡(1+X)).\int_X^1\frac{dx}{x(1+x)}=\bigl[\ln x-\ln(1+x)\bigr]_X^1=-\ln 2-\bigl(\ln X-\ln(1+X)\bigr).

Combine.
I(X)=ln⁡X1+X−ln⁡X+ln⁡(1+X)−ln⁡2=−Xln⁡X1+X+ln⁡(1+X)−ln⁡2.I(X)=\frac{\ln X}{1+X}-\ln X+\ln(1+X)-\ln 2=-\frac{X\ln X}{1+X}+\ln(1+X)-\ln 2.
(The simplification uses 11+X−1=−X1+X\frac{1}{1+X}-1=-\frac{X}{1+X}.)

Limit as X→0+X\to0^+. Since Xln⁡X→0X\ln X\to0 and ln⁡(1+X)→0\ln(1+X)\to0, we get
I=lim⁡X→0+I(X)=0+0−ln⁡2=−ln⁡2.I=\lim_{X\to0^+}I(X)=0+0-\ln 2=-\ln 2.

Remark. The naive computation
∫01ln⁡x(1+x)2 dx=?[−ln⁡x1+x]01+∫01dxx(1+x)\int_0^1\frac{\ln x}{(1+x)^2}\,dx\overset{?}{=}\left[-\frac{\ln x}{1+x}\right]_0^1+\int_0^1\frac{dx}{x(1+x)}
is meaningless because ln⁡x\ln x is not defined at 00 and ∫01dxx\int_0^1\frac{dx}{x} diverges. The correct procedure is to integrate over [X,1][X,1] with X>0X>0 and let X→0+X\to0^+.