Stats/Probability Theory REALLY

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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
 
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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.
 
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 won't 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?
 
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