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 P: 19 I don't get it. Why there's $$\frac{\partial L}{\partial v^2}$$? If we expand function in Taylor series (taking $$\vec{\epsilon}$$ as an independent variable) there should be only derivatives in respect to that var (epsilon). -edit- I think I got it - from mean value theorem we have: $$\frac{f(x+h) - f(x)}{h} \approx f'(x), h \to 0$$, so: $$L( (v+\epsilon)^2 ) \approx L (v^2) + \frac{\partial L (v^2)}{\partial v^2} (v^2)' \epsilon$$ :)