I would appreciate a little help here.
Let $X$ and $Y$ be continuous random variables with a joint pdf of the form $$f(x,y)=K(x+y)$$ $$0< x <y<1$$
Find $K$. I solved it. It turns out to be $2$.
Find the marginals. Again, I found those Find the joint CDF, $F(x,y)$. This is the part I am stuck on. I used the CDF definition but I still get answers that are different from the book.