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Homework Help: Marginal Probability Function

  1. Nov 23, 2008 #1
    I have this question which I cannot seem to solve:
    The joint probability mass function p(x, y) of two discrete random variables X and Y is given by.
    p(x,y) = ([5^x][7^y][e^-5])/x!(y-x)!
    x and y are non-negative integers and x <= y
    (i) Find the marginal probability mass functions of X and Y.
  2. jcsd
  3. Nov 23, 2008 #2
    You would have to "integrate out" the dependence on the second variable. Explicitly

    p(x)=\sum_{y\geq x}p(x,y)
    p(y)=\sum_{x\leq y}p(x,y)
    By the way, you joint pmf doesn't sum to one, but to e37.
  4. Nov 23, 2008 #3
    but how do i integrate the factorials?
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