f(x,y) = 6/7(x+y)^2 for 0<=x<=1 and 0<=y<=1

a.Find the marginal densities of X and Y. .

b.By integrating over the appropriate regions, find:

i)P(X>Y)

ii)P(X+Y)<=1

iii)P(x>=1/2)

For this one, I got the answer for part(i). That is 1-P(X<=Y).

but even for that, why do we take limits for the double integration as 0 to 1 and then x to 2?