设随机变量X,Y有E(X)= E(Y)=0, E(XY)= -1/8, 则Cov(X,Y)=____(a/b)
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设随机变量X,Y有E(X)= E(Y)=0, E(XY)= -1/8, D(X)= D(Y)=3/4, 则ρXY =____(a/b)
设随机变量X,Y有D(X)= 9,D(Y)= 4,ρXY =1/3,则Cov(X,Y)=____
设随机变量X,Y有E(X)=1/4, E(Y)= 1/6, E(XY)= 1/12, 则Cov(X,Y) =____(a/b)
设随机变量X和Y有E(X)=0.6,E(Y)=0.5,E(XY)=0.28,则Cov(X,Y)=____