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Homework Help: Stats/Probability Theory REALLY need help

  1. Nov 28, 2005 #1
    Stats/Probability Theory REALLY need help :(

    I really need some help.

    I have a joint probability mass function...

    P(X=x, Y=y) = (20Cx,y,20-x-y)(0.1)^x(0.3)^y(0.5)^(20-x-y)
    where 0</= x + y </= 20

    b) show that the margninal distribution of X is binomial
    c) find E[XY]

    Somebody please help me.

    Thanks
     
    Last edited by a moderator: Nov 28, 2005
  2. jcsd
  3. Nov 28, 2005 #2

    HallsofIvy

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    Science Advisor

    I have no idea what "(20Cx,y,20-x-y)" means. I might guess that 20Cx is the binomial coefficient but since you haven't said that x and y must be integers it might not be.
     
  4. Nov 28, 2005 #3
    oh sorry that means choose

    P(X=x, Y=y) = [(20)C(x,y,20-x-y)](0.1)^x(0.3)^y(0.5)^(20-x-y)
    where 0</= x + y </= 20

    and C means chosse...

    Also, this question is about a DISCRETE random variable, so integrals wont work.

    Over the holidays, i have to learn how to use that "Tex" programme or whatever its called... by the way, where could i actually get a tutorial on how to use that programme?
     
    Last edited by a moderator: Nov 28, 2005
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