Simply include the MathJax between dollar signs ($
), like so:
blah blah $\sum_{x=0}^{100}(x)$ blah
blah blah $\sum_{x=0}^{100}(x)$ blah
If you want the MathJax to be displayed as its own block, rather than inline, use two dollar signs:
$$\sum_{x=0}^{100}(x)$$
$$\sum_{x=0}^{100}(x)$$
For a comprehensive tutorial, see this post on meta.Mathematicsthis post on meta.Mathematics.