$\newcommand{\+}{^{\dagger}} \newcommand{\angles}[1]{\left\langle\, #1 \,\right\rangle} \newcommand{\braces}[1]{\left\lbrace\, #1 \,\right\rbrace} \newcommand{\bracks}[1]{\left\lbrack\, #1 \,\right\rbrack} \newcommand{\ceil}[1]{\,\left\lceil\, #1 \,\right\rceil\,} \newcommand{\dd}{{\rm d}} \newcommand{\down}{\downarrow} \newcommand{\ds}[1]{\displaystyle{#1}} \newcommand{\expo}[1]{\,{\rm e}^{#1}\,} \newcommand{\fermi}{\,{\rm f}} \newcommand{\floor}[1]{\,\left\lfloor #1 \right\rfloor\,} \newcommand{\half}{{1 \over 2}} \newcommand{\ic}{{\rm i}} \newcommand{\iff}{\Longleftrightarrow} \newcommand{\imp}{\Longrightarrow} \newcommand{\isdiv}{\,\left.\right\vert\,} \newcommand{\ket}[1]{\left\vert #1\right\rangle} \newcommand{\ol}[1]{\overline{#1}} \newcommand{\pars}[1]{\left(\, #1 \,\right)} \newcommand{\partiald}[3][]{\frac{\partial^{#1} #2}{\partial #3^{#1}}} \newcommand{\pp}{{\cal P}} \newcommand{\root}[2][]{\,\sqrt[#1]{\vphantom{\large A}\,#2\,}\,} \newcommand{\sech}{\,{\rm sech}} \newcommand{\sgn}{\,{\rm sgn}} \newcommand{\totald}[3][]{\frac{{\rm d}^{#1} #2}{{\rm d} #3^{#1}}} \newcommand{\ul}[1]{\underline{#1}} \newcommand{\verts}[1]{\left\vert\, #1 \,\right\vert} \newcommand{\wt}[1]{\widetilde{#1}}$ \begin{align}&\color{#c00000}{\int_{0}^{\infty} {\arctan\pars{ax} \over x\pars{1+x^{2}}}\,\dd x} =\half\,a\verts{a}\int_{-\infty}^{\infty} {\arctan\pars{x} \over x\pars{x^{2} + a^{2}}}\,\dd x \\[3mm]&=\half\,a\verts{a}\int_{-\infty}^{\infty}\ \overbrace{{\ic \over 2}\,\ln\pars{1 - \ic x \over 1 + \ic x}} ^{\ds{=\ \arctan\pars{x}}}\ {1\over x\pars{x^{2} + a^{2}}}\,\dd x =-\,\half\,a\verts{a}\,\Im\int_{-\infty}^{\infty} {\ln\pars{1 - \ic x} \over x\pars{x^{2} + a^{2}}}\,\dd x \end{align}
Establecer $\ds{t \equiv 1 - \ic x\quad\imp\quad x = \pars{t - 1}\ic}$ : \begin{align}&\color{#c00000}{\int_{0}^{\infty} {\arctan\pars{ax} \over x\pars{1+x^{2}}}\,\dd x} =-\,\half\,a\verts{a}\,\Im\int_{1 + \infty\ic}^{1 - \infty\ic} {\ln\pars{t} \over \pars{t - 1}\ic\bracks{-\pars{t - 1}^{2} + a^{2}}}\,\ic\,\dd t \\[3mm]&=-\,\half\,a\verts{a}\,\Im\color{#00f}{% \int_{1 - \infty\ic}^{1 + \infty\ic}{\ln\pars{t}\over \pars{t - 1}\pars{t - r_{-}}\pars{t - r_{+}}}\,\dd t} \quad \mbox{where}\quad r_{\pm} = 1 \pm \verts{a} \end{align}
Ahora, evaluaremos el $\ds{\color{#00f}{\mbox{'blue integral'}}}$ . Tomamos la " $\ds{\ln}$ -corte de rama" a lo largo del negativo $\ds{t}$ -semi-eje y cerrar el contorno en un semicírculo " a la derecha " $\ds{\pars{~t > 1~}}$ : \begin{align}&\color{#c00000}{\int_{0}^{\infty} {\arctan\pars{ax} \over x\pars{1+x^{2}}}\,\dd x} =-\,\half\,a\verts{a}\,\Im\bracks{-2\pi\ic\, {\ln\pars{r_{+}} + \ic 0 \over \pars{r_{+} - 1}\pars{r_{+} - r_{-}}}} \\[3mm]&=\pi\,a\verts{a}\,\bracks{% {\ln\pars{1 + \verts{a}} \over \verts{a}\pars{2\verts{a}}}} \end{align}
$$ \color{#66f}{\large\int_{0}^{\infty} {\arctan\pars{ax} \over x\pars{1+x^{2}}}\,\dd x ={\pi \over 2}\,\sgn\pars{a}\ln\pars{1 + \verts{a}}} $$