x

Mathematical expectations: for example

=

[∑(I = 1->； n)

xi]

/

n

( 1)

x

Variance of

:D(x)

=

[∑(I = 1->； n)

(xi

-

Ex)？ ]

/

n

(2)

x

The variance of D(x) is also equal to the mean square value of d (x) = X

-

The square of the mean value Ex of x (Ex)? ,

Namely: D(x)

=

[∑(I = 1->； n)

(xi)？ ]

/

n

-

(Ex)？

(3)