Evidently you use backslashes for everything (Is this a latex-specific thing, or a mathjax-specific thing?)

Turns into

\[

\left( \sum_{k=1}^n a_k b_k \right)^{\!\!2} \leq

\left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)

\]

Inline LaTeX
You can use LaTeX inside of a paragraph as well by surrounding it by $

$<latex>$

$.

The quick brown fox $

$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$

$ jumps over the lazy red dog.

turns into

The quick brown fox $$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$ jumps over the lazy red dog.

Conclusion
If there's anything I'm leaving out (likely) let me know!