There are two ways
In the html editor just type LaTex code between symbols \( … \) (inline).
If you wish to have mathematical symbols in one line per line (display math) just type LaTex code between symbols \[ … \] or $$ … $$
Example:
\[S(f)(t)=a_{0}+\sum_{n=1}^{+\infty}a_n \cos(n \omega t)+b_n \sin(n \omega t)\]
For further convenience, click in the editor icon “Latex” (see image below).
A second math editor will pop up. Just choose the desired math symbol and the latex code will be created automatically.
For further examples take a look at:
You can type chemical symbols using MathJax
Type latex code between symbols \( … \)
if you want one line per line type code between \\[ … \\] or \$\$ … \$\$.
Type chemistry symbols using \ce
For further examples take a look at: