I also found this problem when I was doing the problem, and it was actually very simple to think about it. The condition of probability density is that it should be greater than zero. If f(x, y) = AE, it is easy to know that e cannot be less than zero, so the hidden condition A > 0 is obtained. In the following formula, if a is less than 0, it does not meet the meaning of the question, and the order of x and y should be interchanged for derivation.