T8 数字特征 方差公式

$$\begin{array}{c}D(X)=E[(X-E(X){)}^2]\\ \mathrm{Cov}(X,Y)=E[(X-E(X))(Y-E(Y))]\\ D(aX+bY)=a^{2}D(X)+b^{2}D(Y)+2ab\mathrm{Cov}(X,Y)\end{array}$$