$\displaystyle\int_0^\infty\frac{e^{-2x}x}{(1+e^{-x})^4} \mathrm{d}x=\frac16\left(\ln2-\frac14\right)$. But I don’t know how the calculation went wrong

calculusimproper-integralsintegration

Let $e^{-x}=:t$,
get $\def\dd{\mathrm{d}}
x=-\ln t,\, \dd x = -\dfrac1t \dd t;\; t\colon1\!\backsim\!0$
,
hence
$$
\def\dd{\mathrm{d}}
\begin{align*}
I
&= \int_1^0 \frac{t\ln t}{(1+t)^4}\dd t \\
&= \frac13\int_0^1 t\ln t\,\dd(1+t)^{-3} \\
&= \underbrace{\frac13\left.\frac{t\ln t}{(1+t)^3}\right|_0^1}_{\kern.48em=~0}
+\frac13\int_1^0 (1+\ln t)(1+t)^{-3}\dd t \\
&= \frac16\int_0^1 (1+\ln t)\,\dd(1+t)^{-2} \\
&= \frac16\,\underbrace{\!\left.\frac{1+\ln t}{(1+t)^2}\right|_0^1}_{=\raise{-0.05em}{\large\frac14}-1{\color{#F03}{-\ln0}}}
+\frac16\int_1^0 \frac{\dd t}{t(1+t)^2}.
\end{align*}
$$

 (The part marked in $\color{#F03}{\text{red}}$ is exactly the strange and possibly wrong part)

As to $\displaystyle I_1 = \int_1^0 \frac{\mathrm{d}t}{t(1+t)^2}$,
let $t=\dfrac1u$,
get $\def\dd{\mathrm{d}}
u=\dfrac1t,\, \dd t = -u^{-2}\dd u;\; u\colon1\!\backsim\!\infty$
,
hence
$$
\def\dd{\mathrm{d}}
\begin{align*}
I_1
&= \int_\infty^1 \frac{u}{(1+u)^2}\dd u \\
&= \int_\infty^1 \frac{(1+u)-1}{(1+u)^2}\dd u \\
&= \int_\infty^1 \frac{\dd(1+u)}{1+u} – \int_\infty^1 \frac{\dd(1+u)}{(1+u)^2} \\
&= \biggl.\ln(1+u)\biggr|_\infty^1 + \left.\frac1{1+u}\right|_\infty^1 \\
&= \ln2 {\color{#F03}{-\ln(1+\infty)}} + \frac12.
\end{align*}
$$

Therefore
$$
\begin{align*}
I
&= \frac16\left(\frac14-1-\ln0 + \ln2-\ln(1+\infty)+\frac12\right) \\
&= \frac16\left(\ln2-\frac14\right) {\color{#F03}{-\frac16\ln(0\cdot\infty)}}.
\end{align*}
$$

Please correct me, thank you!
(And it would be great if there was a faster way to solve it!)

Best Answer

To avoid undefined limits, integrate by parts instead as follows

\begin{align*} \int_1^0 \frac{t\ln t}{(1+t)^4}\ dt = &\ \frac16 \int_0^1\ln t\,d\left( -\frac{t^2(3+t)}{(1+t)^3}\right) \\=& \ \frac16 \int_0^1 \frac1{1+t} +\frac1{(1+t)^2} -\frac2{(1+t)^3}\ dt\\= &\ \frac16\ln2 -\frac1{24} \end{align*}