Statistics

$\frac{1}{n}\sum x_i^2 - (\frac{1}{n}\sum x_i)^2$

$\frac{1}{n}\sum x_i - (\frac{1}{n}\sum x_i^2)^2$

$\sum x_i^2 - (\sum x_i)^2$

$\frac{1}{n^2}\sum x_i^2 - \sum x_i$