No, I'm not on about that and of course I've read the textbook first =P
y = 0, f'(x) = 0 - What do? All the textbook says here is how both the methods of finding the gradient (2y dy/dx = f'(x) or 2y = f'(x) dx/dy) both produce 0. An example is question 6 of exercise 12 in the textbook. Maybe I'm just being stupid, but I can't see how they got sqrt(6) as the answer. I am only just learning this chapter now, so yeah...