To find P{X + Y > 3}, where X & Y are jointly distributed random variables, I set the limits of integration to 0-to-1 for dx and 3-x to 5 for dy. This seems reasonable given

0 < x < 1

1 < y < 5

Now, for another question, where I find P{X + Y < 1}, I am told the limits of integration should be from 0 to 1-x for dy and 0-to-1 for dx. (Thinking)

I don' see why,

wirefree