# Thread: Help with a density function

1. ## Help with a density function

Question:
Suppose that random variables X and Y have a joint density function given by

$f_{X,Y} (x,y)=\left\{\begin{array}{cc}x+y,&\mbox{ for }
0 \leq x\leq 1, 0\leq y\leq 1\\0, & \mbox{ otherwise } \end{array}\right.
$

$a) \mbox {Find the density functions of X and Y,} i.e., f_X(x) \mbox {and} f_Y(y)$
$b)\mbox {Find E[X] and }Var[Y]$

What I have done so far:
Frankly I have been staring at this problem for a while. I'm not sure where to start. I cannot apply the following definition for joint probability functions
$f_{X,Y}(x,y)=f_X(x)\bullet f_Y(y)$ as the probability function is not independent. Anyone care to point me in the right direction? Once I could get it figured out, I can solve for b.

PS. Please don't post any solutions as it defeats the whole purpose of asking for help, I won't learn anything that way.

2. Something just hit me, in order to find f(x) do I compute the density function by integrating in respect to dy?

3. $f_X(x)=\displaystyle \int_y f_{X,Y}(x,y) dy$

$f_{Y}(y)=\displaystyle \int_x f_{X,Y}(x,y) dx$

After you have the marginal distributions, you can easily find the mean and variances of each.

4. Originally Posted by harish21
$f_X(x)=\displaystyle \int_y f_{X,Y}(x,y) dy$

$f_{Y}(y)=\displaystyle \int_x f_{X,Y}(x,y) dx$

After you have the marginal distributions, you can easily find the mean and variances of each.
Thanks a lot! It confirms what I said!