1. The problem statement, all variables and given/known data Quote: " PDE: ∂u/∂x + ∂u/∂y = 0 The general solution is u(x,y) = f(x-y) where f is an arbitrary function. Alternatively, we can also say that the general solution is u(x,y) = g(y-x) where g is an arbitrary function. The two answers are equivalent since u(x,y) = g(y-x) = f[-(x-y)] " I don't see why the two different representations above [u(x,y) = f(x-y) and u(x,y) = g(y-x)] would describe exactly the SAME general solution. Why can we freely switch the order of x and y? Also, I don't understand why u(x,y) = g(y-x) = f[-(x-y)]. 2. Relevant equations N/A 3. The attempt at a solution I was thinking of odd and even functions? But I don't think the quote is meant to restrict only to these special functions... Thanks for any help!