For x∈R^n define g:R->R by g(t)=f(tx) g'(t)?

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The discussion centers on the differentiation of the function g(t) defined as g(t) = f(tx) using the chain rule. The participants clarify that the correct application involves recognizing u(t) = tx as a function of t, leading to g(t) = f(u(t)). The derivative dg(t)/dt is correctly expressed as (df/du)(du/dt), emphasizing the importance of distinguishing between the chain rule and the product rule in this context. The need for clear definitions of the variables, particularly whether x is a constant vector and the nature of the function f, is also highlighted.

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precondition
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Have to use chain rule, so I was thinking,
if we let u(t)=tx where x is the vector,
then g(t)=f(u(t)) so dg(t)/dt=d(f u)(t)/dt=(df(t)/dt)u(t)+f(t)(du(t)/dt)
but then I don't know what to do...
 
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Please state the problme clearly. Is x a constant vector? What is f?

What you have stated is not the chain rule- it looks more like the product rule. f(u(t)) is not "f u".

dg(t)/dt= (df/du)(du/dt).
 

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