- #1
JulienB
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Homework Statement
Hi everybody! I'm preparing a maths exam so I run through a lot of problems, and I am unsure whether I tackled this one correctly or not:
Prove that for every differentiable function ##f: \mathbb{R}^3 \to \mathbb{R}## and for every point ##x \in \mathbb{R}^3## the following statement is true:
##\lim\limits_{t \to 0} \frac{f(x+t \nabla f(x)) - f(x)}{t} \geq 0##
Homework Equations
I have the following definitions:
##f'(x) = \frac{\partial (f_1, ..., f_m)}{\partial (x_1, ..., x_n)} (x)##
Directional derivative: ##f'(x)v = \lim\limits_{t \to 0} \frac{1}{t} (f(x + tv) - f(x))##
The Attempt at a Solution
So what I did was pretty straightforward:
##\lim\limits_{t \to 0} \frac{f(x + t \nabla f(x)) - f(x)}{t} = f'(x) \nabla f(x)##
##= [\partial_1 f(x) \partial_2 f(x) \partial_3 f(x)] \cdot [\partial_1 f(x), \partial_2 f(x), \partial_3 f(x)]##
##= (\partial_1 f(x))^2 + (\partial_2 f(x))^2 + (\partial_3 f(x))^2 \geq 0##
Is that legit? Not sure about my handling from ##f'(x)##. Should I have used the total differential definition? Would I obtain the same result? I'm still unclear about total derivatives :)Thanks a lot in advance for your answers.Julien.