# Suppose that f is a differentiable real function in

1. Jan 31, 2012

### Jamin2112

1. The problem statement, all variables and given/known data

Suppose that f is a differentiable real function in an open set E (which is a subset of) ℝn, and that that f has a local maximum at a point x in E. Prove that f'(x)=0.

2. Relevant equations

Definition. Suppose E is an open set in ℝn, f maps E into ℝm, and x is an element of E. If there exists a linear transformation A of ℝn into ℝm such that

limh0 |f(x+h)-f(x)-Ah|/|h|=0,

then f is differentiable at x, and we write

f'(x)=A.​

3. The attempt at a solution

f has a local max at x, so there exists a δ>0 such that f(y)≤f(y) for all y at which |y-x|<δ. Let h=x+y. Whenever |h|≤δ we have f(x)≥f(x+h) or f(x)≥f(x-h).

Now I need to use some algebraic cut-and-paste wizardry to end up with |f(x+h)-f(x)|/|h|. Could I please get an inconspicuous suggestion from the peanut gallery here?

2. Feb 1, 2012

### jake929

your problem statement includes f'(x)=0 your relavent equation has a similar form of f'(x)=A, if you show that A = 0 at a local maxima then you should have your answer. Hint: Make sure that you understand the physical meaning of a local maxima.

3. Feb 1, 2012

### Jamin2112

Did not the first sentence of my attempt capture the concept of a local maxima?

4. Feb 1, 2012

### jake929

I will paraphrase your problem statement and maybe that will help you understand what I mean. The problem is asking you to prove that the slope of a line at a local maxima is 0 using the fundamental theorem of calculus. This is something that will be assumed for later proofs. If you look at the fundamental theorem it will show an equation that looks like what you have seen in pre-calculus classes for calculating the slope (y2-y1)/(x2-x1) = slope. Try and show that the fundamental theorem yields a slope of zero at the maxima. Post your work if you want further help.

5. Feb 1, 2012

### HallsofIvy

Staff Emeritus
jake929, what you are suggesting is valid only in one dimension.

6. Feb 1, 2012

### jake929

Yes, and even though the problem statement is in two dimensions the definition is in one