I know that the variance formula is $\sigma^2 = \frac{ \left( x_1 - \bar{x} \right) ^2 + \left( x_2 - \bar{x} \right) ^2 + \dots + \left( x_n - \bar{x} \right) ^2 }{n}$ Where $\sigma^2$ is the variance; $x_1,\ x_2,\ \dots,\ x_n$ are the statistical data, and $n$ is the number of data.
My question is: how can I expand that formula to get this equivalent one:
$ \sigma^2 = \frac{x_1^2 + x_2^2 + \dots + x^2_n}{n} -\bar{x}^2$
?