1. The problem statement, all variables and given/known data Consider the function of two variables: u(x,y) = f(x-y) + g(x+(1/3)y) where f(s) and g(t) are each arbitrary functions of a single variable. Using the change of variables: s = x-y t = x-(1/3)y use the chain rule to determine the first and second derivatives of u with respect to x and y in terms of derivatives of f and g. Hence, show that the second derivatives satisfy u_xx = 2u_xy + 3u_yy where u_xx is the second derivative of u with respect to x, etc. 3. The attempt at a solution My attempt, along with the original question paper, is attached as a PDF. It looks very fiddly but I have attempted the question a few times and still can’t satisfy the last equation.