Mathematical expectation of x: ex = [∑ (I =1->; xi] / n ( 1)

Variance of x: d (x) = [∑ (I =1->; n) (xi - Ex)？ ] / n (2)

The variance of x: D(x) is also equal to: d (x) = the mean square of x-the square of the mean of x (Ex)? ,

Namely: d (x) = [∑ (I =1->; n) (xi)？ ] / n - (Ex)？ (3)