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Answers for “点积公式如何推广到高维?”

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Understand why

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The directional derivative formula a*(∂f/∂x) + b*(∂f/∂y) structurally matches the definition of a dot product between the vector [a,b] and the vector [∂f/∂x, ∂f/∂y]. Since the second vector is defined as the gradient ∇f, the expression simplifies to w·∇f.

Conditions: The function f(x,y)f(x,y) is differentiable.; The direction vector is w=[a,b]w=[a,b].; The gradient ∇f = [∂f/xf/x, ∂f/yf/y] exists.

Meet the concept

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The directional derivative in the direction of a vector w is equal to the dot product of w and the gradient vector ∇f. The gradient vector contains the partial derivatives of the function as its components.

Conditions: The function f(x,y)f(x,y) is differentiable.; The direction vector is w=[a,b]w=[a,b].; The gradient ∇f = [∂f/∂x, ∂f/∂y] exists.

Know when to use it

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Yes, the dot-product formula w·∇f generalizes naturally to higher dimensions. If the input has five variables, both the direction vector and the gradient vector simply expand to have five components, maintaining the same structural relationship.

Conditions: The function is differentiable in higher dimensions.; The direction vector and gradient are defined in the same dimensional space.