This one uses a Javascript to load MathJax direct. Seems easier than the plug-in.

In equation 1, we find the value of an

interesting integral:

$$\int_0^\infty \frac{x^3}{e^x-1}\,dx = \frac{\pi^4}{15}$$

or this:

\(\int_0^\infty \frac{x^3}{e^x-1}\,dx = \frac{\pi^4}{15}\)

or this:

$$\mathcal{\int_0^\infty \frac{x^3}{e^x-1}\,dx = \frac{\pi^4}{15}}$$