In just the XY plane, a derivative is the slope of the tangent line.
How does it work when you have a 3D shape? I recall 3D math, and the Z-axis. Was this Calc 3?
Let's say you have a "mound". Then a "tangent" to the mound at a point (x,y) will be a plane, right? Is the derivative of a 3d shape a 2d plane? How do you represent this plane?
Anyone have a basic link that will explain the general concepts I am broaching?