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Homework Help: Statistics: Multivariate Dist/Functions of RVs

  1. Nov 27, 2011 #1
    1. The problem statement, all variables and given/known data

    1) A point (X1,X2,X3) are is chosen at random. X1-3 are uniform distributions across the interval [0,1]
    Determine P[(X1-.5)2 + X2-.5)2 + X3-.5)2 ≤ .25]

    2) X and Y are random variables with the joint pdf: f(x,y) = 2(x + y) for 0≤x≤y≤1, 0 otherwise.
    Find the pdf of Z = X + Y

    2. The attempt at a solution

    1) So I know the pdf for each one of the random variables is f(x) = 1 when 0≤x≤1
    Imagining it in a grid, it would be the volume of the sphere with radius .5 centered at (.5,.5,.5). I am uncertain where to begin applying the pdf of each of the random variables to the graph of the probability needed to be found.

    2) I know this has appeared once in another topic, but I can't figure out the bounds of this problem (if I'm not messing up elsewhere).
    So the graph envisioned would be the points that lie above the line y=x, bounded by y <= 1, x >=0.
    So what I would do from this point is attempt to find the solution for (1-x to 1)∫f(x,z-x)dx = (1-x to 1)∫2zdx = 2z -2z + 2zx = 2zx. What is wrong with this reasoning/where do I approach this problem after this?
    Last edited: Nov 27, 2011
  2. jcsd
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