Practice: the second-step error

Suppose we start at (a, b) and use the tangent plane at (a, b) to approximate f(a + Δx, b + Δy). We move in two coordinate steps:

  1. (a, b) → (a + Δx, b)
  2. (a + Δx, b) → (a + Δx, b + Δy)

Define error as actual height change − estimated height change.

Which formula gives the error in Step 2?

Choose one formula