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How does the total differential dz differ from the actual increment Δz in terms of their geometric measurement?

The actual increment Δz\Delta z measures the true change in height on the curved surface, calculated as the new function value minus the reference value. The total differential dzdz measures the height change on the tangent plane for the same input displacement. Both are measured relative to the reference height, but Δz\Delta z follows the surface curvature while dzdz follows the linear approximation.

Conditions

  • The input is displaced by (Δx,Δy)(\Delta x, \Delta y) from a reference point (x0,y0)(x_0, y_0).
  • The function is differentiable at the reference point.

Reasoning, step by step

  1. Identify the reference height f(x0,y0)f(x_0, y_0).
  2. Calculate the new height on the surface: f(x0+Δx,y0+Δy)f(x_0+\Delta x, y_0+\Delta y).
  3. Find the actual increment: Δz=f(x0+Δx,y0+Δy)−f(x0,y0)\Delta z = f(x_0+\Delta x, y_0+\Delta y) - f(x_0, y_0).
  4. Calculate the height on the tangent plane using partial derivatives: dz=fx(x0,y0)Δx+fy(x0,y0)Δydz = f_x(x_0, y_0)\Delta x + f_y(x_0, y_0)\Delta y.
  5. Compare the two: Δz\Delta z is the surface change, dzdz is the plane change.

Example

The script states: 'The actual increment Δz is the new function value minus its reference value. The total differential dz is the tangent plane’s height change for the same input displacement. Both are measured relative to the reference height.'

Common misconceptions

  • Thinking that Δz\Delta z and dzdz are identical for any displacement.
  • Believing that dzdz represents the absolute height on the tangent plane rather than the change in height.
  • Confusing the geometric object being measured (curved surface vs. flat tangent plane).

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