1. The problem statement, all variables and given/known data Solve: y' = cos(x-y) 2. Relevant equations 3. The attempt at a solution Using x-y=t and solving the integrals, I get that the general solution is: -cot(0.5(x-y)) = x + c which is correct, but there's another solution which is x-y=2πk, but I don't understand why. In the integral I get that it is undefined for 1-cost=0. Meaning it is undefined for cost = 1, or cos(x-y) = 1. Plugging it in the original problem to see if it satisfied it gives me y' = 1. How can I know if this is true for all y (I guess it is)... I didn't exactly understand what do I need to do here.