let me draw another surface, I won't use the one that I had already drawn on — if we draw a surface, and for a very small change in u, and we're not gonna think about the rate; we're just thinking about kind of the change in r, you're going in that direction, so this thing, if that thing looks like this, this is actually kind of a distance moved on the surface, because it's just, it's not, remember, this isn't the derivative ∂r/∂u, this is the differential ∂r. 