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## Homework Statement

I have a joint pdf f_{XY}(x,y) = (2+x+y)/8

and I have to find cov(x,y)

## Homework Equations

Now I know the formula is cov(X,Y)=E((X-mu)(Y-v)) where E(X)=mu and E(Y)=v

Well I found that formula on wikipedia, but it doesn't make sense to me because if E(X)=mu then doesn't E(X-mu) equal zero?

Well thats my main problem, I dont know how to use the formula.

## The Attempt at a Solution

I've worked out the distribution table to see if that would help

. . . . y

. . . -1___0___1

. -1 0, 1/8, 1/4 (X=-1) =3/8

x 0 1/8, 1/4, 3/8 (X=0) =3/4

. .1 1/4, 3/8, 1/2 (X=1)=1/18

(Y=-1) = 3/8 (Y=0)=3/4 (Y=1) = 1/1/8

Sorry its not in LaTex I couldn't remember how to start the code.

I've also worked out that the var(Y) = 1 if that is any help? I have an exam on this very soon and I really want to be able to get a hold of all the main concepts... thanks for any information