I was integrating the Planck distribution and came across this integral:
$$\int_0^\infty \frac {1}{x^5\cdot \left(e^{\frac{a}{x}}-1\right)} dx$$
From this page: http://w.astro.berkeley.edu/~echiang/rad/ps1ans.pdf the given solution is
$$\int_0^\infty \frac {1}{x^5\cdot \left(e^{\frac{1}{x}}-1\right)}dx = \frac {\pi^4}{15} $$
and I could analytically work out that the general solution is
$$\int_0^\infty \frac {1}{x^5\cdot \left(e^{\frac{a}{x}}-1\right)}dx = \frac {\pi^4}{15a^4} $$
I was wondering how one could prove this, and if my general solution is accurate or not.