1. The problem statement, all variables and given/known data When x=! -1, y=! -1, x=! -y then x and y are two numbers so that 1/(x+1) + 1/(x+y) + 1/(y+1)... is an arithmetic serie. Show that then also x2 + 1 + y2.... must be an arithmetic serie. 3. The attempt at a solution I tried to find the differentials between each number in the first line, then make them equal each other but I really didn't get anywhere. I figured out though that if x=y=1 then both of the series are arithmetic. Anyways can I get some help pls?