t**k
发帖数: 10 | 1 E{X+Y} = E{X} + E{Y} for any X & Y, no matter they are independent or not.
so, E{Z} = E{X} - E{Y} = 1/6 - 1/2 =-1/3 | s********e
发帖数: 31 | 2 Var(Z)=Var(X-Y)=Var(X)+Var(Y)-2Cov(X,Y)
since X Y are independent, Cov(X,Y)=0
Var(X)=E(X^2)-(E(X)^2)=5/36
Var(Y)=E(Y^2)-(E(Y)^2)=1/4
thus, Var(Z)=14/36=7/18 |
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