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The Taylor expansion of \( f(x,y) \) about \( (a,b) \) has first-order terms

  • A.\( (x-a)(y-b)f_{xy} \)
  • B.\( (x-a)f_{x} + (y-b)f_{y} \)
  • C.\( f_{x} + f_{y} \)
  • D.\( (x-a)f_{y} + (y-b)f_{x} \)

Answer: B

Each first derivative is weighted by the increment of its own variable, so the linear part is

\[ f(a,b) + (x-a)f_{x}(a,b) + (y-b)f_{y}(a,b). \]

This is the tangent plane to the surface at the point. The pairing that crosses the two derivatives attaches each to the wrong increment, and the term in \\( f_{xy} \\) belongs to the second-order group

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