# Find the joint pdf of X and Y for two uniform distributions

Let X have a uniform distribution on the interval $(0,1)$. Given that X = x, let Y have a uniform distribution on the interval $(0,x+1)$.

Find the joint pdf of X and Y. Sketch the region where $f(x,y) > 0$.

Find fY$(y)$, the marginal pdf of Y. Be sure to include the domain.

I'm not really sure where to start. Is fY$(y)$ just $\frac{1}{(x+1)-0}$ since that's the pdf of a uniform distribution? And I have no idea how to find the joint pdf of X and Y.

The conditional density of $Y$ given $X=x$ is $f_{Y|X}(y|x)=\frac{1}{x+1}1_{0<y<x+1}$, hence the joint density of $X$ and $Y$ is $$f(x,y)=f_{Y|X}(y|x)f_X(x)=\frac{1}{x+1}1_{0<x<1,0<y<x+1}$$
The marginal density can then be obtained by "integrating out" the $x$-variable: $$f_Y(y)=\int f(x,y)\;dx$$
• This is the conditional density of $Y$ given $X=x$. See here for more info: en.wikipedia.org/wiki/Conditional_probability_distribution – carmichael561 Apr 7 '16 at 3:35
• That is a shorthand notation for 1 if $0<y<x+1$, and 0 otherwise. – carmichael561 Apr 7 '16 at 3:41
• I'm a bit confused on the bounds to use on the integral to calculate the marginal density of y. I believe $y$ should be the lower bound, but what is the upper bound? It can't be $x+1$, right? – bhav1001 Apr 7 '16 at 4:05