<t>Function have a rotation symmetry around 0. So, when you integrate the function over a domaine x>0, x<z (x-y=z, y<0) you get 1/8 of the integral over the whole plane.Another way to see the result is to see that events W(1)>0, W(2)-W(1)<0 and abs(W(1))<abs(W(2)-W(1)) are independent, with probabil...