- #1
phiiota
- 29
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Homework Statement
Suppose that X1 and X2 have the joint pmf
[tex]f(x_{1},x_{2})=p^{2}q^{x_{2}},x_{1}=0,1,2,...,x_{2},x_{2}=0,1,2,...[/tex]
with
[tex]0<p<1,q=1-p[/tex]
Homework Equations
The Attempt at a Solution
I'm confused because the expression doesn't have x_1 in it. So usually, if I want to find f1x(1), say, I would add up all the x_2 probabilities where x_1=1 to get my marginal (summing over all values of x_2). But without having an x_1 in my pmf, I don't know how to do this. I'm tempted to say the marginal is just the original pmf, but how would I differentiate between p(x_1=1,x_2=2) from p(x_1=0,x_2=2)?