This forum supports \LaTeX equations between $dollar signs$. The default LaTeX style is the “inline” style which looks like \sum_{k=0}^n {n \choose k} = 2^n , in the corresponding text is
$ \sum_{k=0}^n {n \choose k} = 2^n $
However, if you start and end your equation with $$ on its own beginning and terminating lines as it follows,
$$
\sigma = \sqrt{ \frac{1}{N} \sum_{i=1}^N (x_i -\mu)^2}
$$
You will get
\sigma = \sqrt{ \frac{1}{N} \sum_{i=1}^N (x_i -\mu)^2}
Enjoy beautiful math in all browsers.