设随机变量X,Y有E(X)=2/3,E(Y)=6, E(XY)=4 ,则Cov(X,Y) =____
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设二维随机变量X,Y有D(X)=D(Y)=1, ρXY =0, 则Cov(X,Y) =____
设二维随机变量X,Y有D(X)=D(Y)=1, ρXY =1/2, 则Cov(X,Y) =____
设随机变量X,Y有E(X)=0.8, E(Y)=0.1, E(XY)=0, 则Cov(X,Y)=____
设随机变量X,Y有E(X)=0.8, E(Y)=0.1, E(XY)=0, D(X)=0.16, D(Y)=0.09, 则ρXY =____(a/b)
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