use as raw text "\(\sum_i x_i \le (x_1, x_2, \ldots, x_n )\)". Within the delimiters, rudimentary subscripting, superscripting, greek letters, products, sums, and so on are available by prefixing words by backslashes. Generally, latex characters (e.g. \leftarrow) are available if there are html 4.0 equivalents.
Your mileage may vary...
Examples:
~x ⊆ ‹S› | \( \~\overline{x} \subseteq \langle{}S\rangle \) |
∑i=110 i = 10 × 11/2 . | \( \sum_{i=1}^10 i ~ = ~ 10 \times 11/2 \). |
For a more complete list of the characters available, go here.
Generally, latex commands that take arguments are not supported, (such as \frac) nor are environments such as arrays... The only exception currently is the \overline{} macro.