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Thread: Discrete Random Variable

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    Discrete Random Variable




    Hey all, I have a Discrete Random Variable taking on values x= 1,3,3^2,3^3,...3^m and f(x)=P(X=x)=c/x for some constant c, find value of c.


    How do i do this question im 100% lost

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    Re: Discrete Random Variable

    What needs to be true about f(x) for it to be a valid probability distribution?
    I don't have emotions and sometimes that makes me very sad.

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    Re: Discrete Random Variable

    f(x)>=0 in order to be true.

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    Re: Discrete Random Variable

    That's not the only thing that needs to be true.

    For example is
    f(0) = .5
    f(1) = .5
    f(2) = .5
    f(x) = 0 for any x not equal to 0,1,2

    a valid probability distribution?
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    Re: Discrete Random Variable

    I would say no because f(0)= 0, f(1)=.5 f(2)=1 right ? I dont think im understanding you correctly.

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    Re: Discrete Random Variable

    What? I already defined f(0), f(1), f(2). You can't just change them. So why isn't that a valid pdf? (Hint: Think about the sum)
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    Re: Discrete Random Variable

    I see, its because the values do not add up to 1 right ? (.5+.5+.5+=1.5)

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    Re: Discrete Random Variable

    Exactly. So going back to your original problem we can see that you need to find the value of c that makes the sum over all possible values equal 1.
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    Re: Discrete Random Variable

    I understand now BUT i still dont see how to calculate C if the problem is 3^m, i mean if i had a definitive set i can do it but i dont understand this method..Im thinking C=1 but i dont know if thats correct or not..

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    Re: Discrete Random Variable

    You might want to recall the Geometric series
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    Re: Discrete Random Variable



    in this case a=1, r=3 but i dont know what s and n are?

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    Re: Discrete Random Variable

    Any one ?

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    Re: Discrete Random Variable

    Which series you want to calculate? Maybe write out some terms can help you to figure out the problem.

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    Re: Discrete Random Variable

    Hey all, I have a Discrete Random Variable taking on values x= 1,3,3^2,3^3,...3^m and f(x)=P(X=x)=c/x for some constant c, find value of c.


    Thats all the question asks

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    Re: Discrete Random Variable


    Of course I know the question. I just want to make sure: do you know which geometric series are you aiming at? Can you write that down?

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