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Okay, I have another one here: Suppose that X1,...,X100 are random variables with E(Xi)=100,E(X2i)=10100 If Cov(Xi,Xj)=−1,i≠j what is Var(S), where S=∑i=1100Xi ? I got Var(S) to be 100, which is the correct answer but what good does knowing the covariance do? I tried using it, and it just throws everything off.

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what does mean E(X2i) means E(even) ?
It means \[E(X _{i}^{2})\] Sorry about the notation - it didn't copy and paste correctly.
oh got

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so S is sigma(i)=1100Xi is it right
but it is not the answer because the problem wants var(s) but i dont know what is exact s?
Sorry, \[S=\sum_{i=1}^{100}X _{i}\]
I can't see the right side of what you did.
How'd you get n^2-n?
|dw:1392447809874:dw| How'd you get this part?
all is n^2 so you have to subtract from n (x1 x2 .. xn)
all term in sigma without itself terms
Suppose that (X,Y) is uniformly chosen from the set given by \[0

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