Hi, I just don't get this. I'm to lazy to type inn what I've done, so I just took a picture of my textbook: http://home.no.net/erfr1/images/1.jpg http://home.no.net/erfr1/images/2.jpg You're supposed to end up with the starting point, right? So you can divide the rest by two? Whatever I try, I get zero... I got the solution from mathematica, but don't understand a thing of it. Thanks!