设设随机变量X和Y有ρXY=0.9,若Z=X-0.4,, 则ρYZ =( )

A. 0.9;
B. 0.5;
C. 0.4;
D. 0.81。

已知E(X)= E(Y)=0, D(X)=1,D(Y)=4, ρXY =1/2 , 若Z=aX+Y与Y独立,则a等于( )

A. 2
B. -2
C. 4
D. -4

若X和Y满足D(X+Y)=D(X-Y),则必有( )

A. X和Y独立
B. X和Y 不相关
C. D(Y)=0
D(X+Y) ·D(X-Y)=0

E(X)=1/2 , E(Y)=1/4 E(XY)= 1/4,则Cov(X,Y)= ____(a/b)

已知E(X)= E(Y)=7/6, E(X2) =E(Y2)=5/3, E(XY)=4/3, 则Cov(X,Y)= = ____(a/b)

已知E(X)= E(Y)=7/6, E(X2) =E(Y2)=5/3, E(XY)=4/3, 则ρXY= = ____(a/b)

设随机变量X,Y有E(X)=E(Y)=0,E(X2)=5/8, E(Y2)=17/32, E(XY)=9/16, 则Cov(X,Y)= = ____(a/b)

设随机变量X,Y有E(X)=2/3,E(Y)=6, E(XY)=4 ,则Cov(X,Y) =____