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Homework Help: Covariance of Binomial Random Variables

  1. Nov 8, 2009 #1
    1. The problem statement, all variables and given/known data
    Let X be the number of 1's and Y be the number of 2's that occur in n rolls of a fair die. Find Cov(X, Y)

    2. Relevant equations
    Cov(X,Y) = E(XY) - E(X)E(Y)

    3. The attempt at a solution
    Both X and Y are binomial with parameters n and 1/6. Thus it is easy to find E(X) and E(Y), but for some reason finding E(XY) is really tripping me up. How does one go about finding expected values of products of binomial variables?
  2. jcsd
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