= E{X？ y？ -2XYE(XY)+E？ (XY)}

= E(X？ )E(Y？ )-2E？ (X)E？ (Y)+E？ (X)E？ (Y)

= E(X？ )E(Y？ )-E？ (X)E？ (Y) (2)

If E(X) = E(Y) = 0,

So D(XY) = E(X? )E(Y？ )= D(X)D(Y)，(3)

That is, when X and Y are independent of each other and their mathematical expectations are all zero, the variance d (XY) of the product xy of X and Y is equal to:

D(XY) = D(X)D(Y)。 (4) //: That is the formula (3).