You can use an inline formula \(\forall x \in R\) like this one

$$\forall x \in R$$
\[M = \left( \begin{array}{ccc} x_{11} & x_{12} & \ldots \\ x_{21} & x_{22} & \ldots \\ \vdots & \vdots & \ldots \\ \end{array} \right)\]

Add this script in your page:

<script src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" type="text/javascript"></script>