1. The problem statement, all variables and given/known data Suppose you are standing at the point (-100,-100,360) on a hill that has the shape of the graph of z=500-0.006x2-0.008y2. In what direction should you head to maintain a constant altitude? 2. Relevant equations Duf = ∇f[itex]\bullet[/itex]u formula for directional derivative 3. The attempt at a solution Here's my idea. If the directional derivative at the point specified is zero, then that's equivalent to maintaining constant altitude. Therefore, I must find the corresponding gradient vector ∇f when the directional derivative is zero right? I have no idea how to carry this out however.