I know how to find the directional derivative in the direction of unit vector u, but it doesn't specify what u is so what do I doI am assuming that [imath]\frac{df}{ds}|_u[/imath] means derivative of [imath]f[/imath] over the length [imath]s[/imath] in the direction [imath]u[/imath]. It is easily derived from the gradient and the unit vector [imath]u[/imath] -- do you know how ?
9.b explicitly asks you to find which [imath]u[/imath]'s correspond to the minimum and the maximum derivatives. What is your formula for the directional derivative corresponding to an arbitrary [imath]u[/imath]?I know how to find the directional derivative in the direction of unit vector u, but it doesn't specify what u is so what do I do