Concept: *The Magnitude of a Partial Derivative (editing)

FIXME: Add for Contours: "The magnitude of a partial derivative is proportional to the density of contour lines along the path of the partial derivative."[]FIXME: Add for Surfaces: "The magnitude of a partial derivative is the slope of the surface along the path of the partial derivative."[]FIXME: Add current concept: "All partial derivatives can be found as a slope of a tangent plane" and change to "The Magnitude of a partial derivative a slope of the tangent plane in the direction of the partial derivative FIXME: Direction Language?"