The MathJax syntax is similar to \LaTeX syntax. That logo is itself an example of inline MathJax: $\LaTeX$.
Matrices
\begin{bmatrix} a_{11} & a_{12} \\ a_{21} & a_{22}\end{bmatrix} \times
\begin{bmatrix} b_{11} & b_{12} \\ b_{21} & b_{22}\end{bmatrix}
$$
\begin{bmatrix} a_{11} & a_{12} \\ a_{21} & a_{22}\end{bmatrix}
\times
\begin{bmatrix} b_{11} & b_{12} \\ b_{21} & b_{22}\end{bmatrix}
$$
Complex equations
\widehat{\ell\,}(\theta\,;x)=
\frac1n \sum_{i=1}^n \ln f(x_i\mid\theta)
$$
\widehat{\ell\,}(\theta\,;x)=
\frac1n \sum_{i=1}^n \ln f(x_i\mid\theta)
$$