Density function f(x, y)= 1/a of the joint distribution function of x and y? ，(x，y)∈D，f(x，y)=0，(x，y)？ d .

And when x≥y, x-y =x-y, x.

∴E[ 乸乸乸 X-Y 乸] = ∫ (0, a) dy ∫ (y, a) (x-y) f (x, y) dx+∫ (0, a) dx ∫

For reference.