Expanding a multivariable function in powers of one of the variables?

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SUMMARY

The discussion centers on expanding the Lagrangian function L(v'2) in powers of the small vector ε, as presented in Landau's classical mechanics text. The key result derived is L(v'2) = L(v2) + ∂L/∂(v2) * 2v⋅ε, where v and ε are vectors. This expansion utilizes the first-order Taylor series approximation, highlighting the significance of the partial derivative of L with respect to v2. The explanation sought by the user was found on physics.stackexchange.com, providing clarity on the derivation process.

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So I've started reading Landau's classical mechanics text and I'm at the part where L(v'2) = L(v2 + 2v⋅ε +ε2) and he says to expand it out in powers of ε to get L(v'2) = L(v2) + ∂L/∂(v2) * 2v⋅ε where v and ε are vectors.

Can someone explain how this result is obtained in more detail?
 
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