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.