协方差Cov(X，Y)。
E(Y)=0×0.3+2×0.3+4×0.4=2.2
E(XY)=0×0.3+2×0.2+4×0.4+8×0.1=2.8
COV(X，Y)=E(XY)-E(X)E(Y)=2.8-2.2×1.4=-0.28
协方差Cov(X，Y)。
E(Y)=0×0.3+2×0.3+4×0.4=2.2
E(XY)=0×0.3+2×0.2+4×0.4+8×0.1=2.8
COV(X，Y)=E(XY)-E(X)E(Y)=2.8-2.2×1.4=-0.28
