1. The problem statement, all variables and given/known data f a differentiable real valued function lim h - > 0 of (f(x + ah) - f(x + bh))/h where a,b real numbers 2. Relevant equations definition of derivative lim h-> 0 of (f(x+h) - f(x))/h 3. The attempt at a solution I've picked several functions like x^2 and 1/2x in the first case you get 2xa - 2xb = 2x(a-b) in the latter you get 1/2a - 1/2b = 1/2(a-b) This leads me to suspect f'(x)*(a - b) as the solution, I just have no idea what to do to the limit. I tried working backwards from my solution, but I still am missing something. Any help is appreciated.