How Do You Calculate Marginal Density for fX(x) Given fX,Y(x,y)?

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SUMMARY

The discussion focuses on calculating the marginal density function \( f_X(x) \) from the joint density function \( f_{X,Y}(x,y) = 4x e^{-(x+y)} \) for \( 0 < x < y \). The correct marginal density is derived by integrating the joint density over the appropriate limits, resulting in \( f_X(x) = 4x e^{-2x} \). Participants highlighted the importance of correctly setting the limits of integration to avoid issues with infinity in the calculations.

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Homework Statement



Calculate the marginal density of fx (x).
Fx,y (x,y) = double integral of 4x.e^(x+y) when 0<x<y, 0 otherwise.


Homework Equations





The Attempt at a Solution



i set up the equation as fx (x) = integral from x up to infinity of 4x.e^(x+y) dy

i get 4x [e^(x+y)] from x to infinity,

thats when the problem arises as whatever i do there's always infinity in the question.

I'm told the answer is 4x.e^(-2x)

any help would be really appreciated.
thanks.
 
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are you sure you haven't missed a minus sign in the original distribution>
[tex]f_{X,Y} (x,y)dxdy = 4xe^{-(x+y)}dx dy[/tex]

would make more sense, the distrubution you gave is unbounded
 

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