<s>[INST] Remarkable integral: $\int_0^{\infty} x \left(1 - \frac{\sinh x}{\cosh x-\sqrt 3/2} \right) \mathrm dx= -\frac{13 \pi ^2}{72}$? Numerical evidence suggests that
$$\int_0^{\infty} x \left(1 - \frac{\sinh x}{\cosh x-\sqrt 3/2} \right) \mathrm dx=  -\frac{13 \pi ^2}{72}$$
How can we prove this? I could not find a nice contour in the complex plane to integrate around. Integration by parts also didn't help.
Mathematica finds a very complicated antiderivative in terms of special functions, but this was a contest problem so there must be a 'human' way to calculate it.
As O.L. helpfully pointed out, I had the sign wrong. It is corrected now.
 [/INST] The following is another approach that also involves differentiating under the integral sign.
Let $$I(\theta) = \int_{0}^{\infty} x \left(1- \frac{\sinh x}{\cosh x - \cos \theta} \right) \, \mathrm dx \, , \quad 0 < \theta < \pi. $$
Then $$I'(\theta) = \sin \theta \int_{0}^{\infty} \frac{x \sinh x}{\left(\cosh x - \cos \theta \right)^{2}} \, \mathrm dx.$$
I tried evaluating $I'(\theta)$ using contour integration, but calculating the residues became too tedious.
Instead let $a$ be a positive parameter, and let $$J(\alpha) = \int_{0}^{\infty}\frac{\mathrm d x}{\cosh (\alpha x)- \cos \theta} = \frac{1}{\alpha} \int_{0}^{\infty} \frac{du}{\cosh u - \cos \theta}.$$
Then $I'(\theta) = -\sin (\theta) \, J'(1) $.
To evaluate $J(\alpha)$, let's integrate the function $f(z) = \frac{z}{\cosh z - \cos \theta}$ around a rectangle contour in the upper half of the complex plane of height $2 \pi i $.
We get $$- 2 \pi i \int_{-\infty}^{\infty}  \frac{dt}{\cosh t - \cos \theta} = 2 \pi i \, \left(\operatorname{Res} \left[f(z), i \theta \right]+ \operatorname{Res} \left[f(z), i \left(2 \pi - \theta\right)\right] \right)= 4 \pi i \, \frac{\left(\theta - \pi\right)}{\sin \theta}.$$
Therefore, $J(\alpha) = \frac{1}{\alpha}\frac{\pi - \theta}{\sin \theta} $, which means $I'(\theta) =\pi - \theta $.
Integrating with respect to $\theta$, we get $$I(\theta) =  \pi \theta -\frac{\theta^{2}}{2} + C. $$
To determine the constant of integration, we can evaluate $I(\theta)$ at $\theta = \frac{\pi}{2}$.
$$\begin{align} I(\pi/2) &= \int_{0}^{\infty} x \left(1- \tanh x \right) \mathrm dx \\ &= (1- \tanh x) \frac{x^{2}}{2}\Bigg|^{\infty}_{0} + \frac{1}{2}\int_{0}^{\infty}\frac{x^{2}}{\cosh^{2} x} \mathrm dx  \\ &= \frac{1}{2} \int_{0}^{\infty} \frac{x^{2}}{\cosh^{2} x} \mathrm dx \\ &= 2 \int_{0}^{\infty}x^{2} \,  \frac{e^{-2x}}{(1+e^{-2x})^{2}} \, \mathrm dx \\ &= 2 \int_{0}^{\infty}x^{2} \, \sum_{n=1}^{\infty} (-1)^{n-1} n e^{-2nx} \mathrm dx \\ &= 2 \sum_{n=1}^{\infty} (-1)^{n-1} n \int_{0}^{\infty} x^{2} e^{-2nx} \, \mathrm dx \\ &= \frac{1}{2} \sum_{n=1}^{\infty} (-1)^{n-1} \frac{1}{n^{2}} \\ &= \frac{\pi^{2}}{24}. \end{align}$$
So $C= \frac{\pi^{2}}{24} -\frac{\pi^2}{2} + \frac{\pi^{2}}{8} = -\frac{\pi^2}{3}$, and, therefore, $$I(\theta) = \pi \theta -\frac{\theta^{2}}{2} - \frac{\pi^{2}}{3}. $$
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