How would you prove the closed form solution of this 2-parameter improper integral: [math]\displaystyle\int_0^\infty \frac{1}{x}\{\tan^{-1}{(ax)}-\tan^{-1}{(bx)}\}dx=\frac{\pi}{2} \ln\frac{a}{b},\quad a,b \in \mathbb{R}?[/math]