# Marginal pdf's

• Dec 27th 2009, 05:50 AM
yog782004
Marginal pdf's
plz help to me, actually I cant find out how to got ans.

the pdf is given as
f(x, y) = 2, if (x,y) є R
else 0

there is marginal pdf's of X and Y

= 2y

then

E(X) = integral 0 to 1, x2(1-x)dx = 1/3

now can you describe this stepwise

E(X) = mathemetical expectation

becoz I cant analyse it
• Dec 27th 2009, 06:41 AM
CaptainBlack
Quote:

Originally Posted by yog782004
plz help to me, actually I cant find out how to got ans.

the pdf is given as
f(x, y) = 2, if (x,y) є R
else 0

there is marginal pdf's of X and Y

= 2y

then

E(X) = integral 0 to 1, x2(1-x)dx = 1/3

now can you describe this stepwise

E(X) = mathemetical expectation

becoz I cant analyse it

If the original of this question was in English please post the exact wording.

CB
• Dec 27th 2009, 06:44 AM
matheagle
we need to know where f(x,y)=2
The support cannot be all of R2.
probably some triangle with x<y or y<x
• Dec 27th 2009, 11:32 PM
yog782004
Intigral
Hi Captain
Hi mathengle

Here I attach one you can find it,
actually it is example of my book and cant analyse these calculation
• Dec 28th 2009, 06:27 AM
matheagle
I'm not sure what you want to know.
The defintion of the mean of X is

$\displaystyle E(X)=\int_{-\infty}^{\infty}xf(x)dx$

You first need to find the density and the support (where it is not zero).
Then integrate.

Likewise $\displaystyle E(X^2)=\int_{-\infty}^{\infty}x^2f(x)dx$
• Dec 28th 2009, 11:30 PM
yog782004
thanks mathegle

I get somethink from your suggetion, actualy I m not sure in intrigral operation and calculation, thats y i hav this confusion.