Variance reflects the average deviation of random variables, D(X)=E[X-E(X)]~2. Variance is also a mathematical expectation in essence, which is the mathematical expectation of a special random variable. Attribute: 1, d (c) = 0; 2、D(CX)= C ~ 2 * D(X)； 3、D(X+C)= D(X)； 4. If X and Y are independent, then D(X+ or-Y) = D (X)+D (Y); It's not convenient to type formulas here. These formulas are commonly used, and there are several properties that are rarely used. Please contact again if necessary!