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Critical point 
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#1
Nov1304, 01:55 PM

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let f(x)=sin(1/x)*x^2 for x not 0, and f(0)=0. show that x=0 is a critical point for f which is neither a local minimum, a local maximum, nor an inflection point.
well I tried differentiating this, and got f'=cos(1/x) +2xsin(1/x). to find a critical point i make f'=0. Not sure how to do this. Do I take the limx>0? Any hints or tips would be great 


#2
Nov1304, 02:15 PM

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What you've done here, is to find the derivative of f at all points EXCEPT at x=0!
But you are to find f'(0)... Use the definition of the derivative. 


#3
Nov1304, 05:53 PM

P: 35

k thanks i'll try that



#4
Nov1404, 07:19 AM

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Critical point
If you're a bit unsure what I mean, the definition of f'(0) is:
[tex]f'(0)=\lim_{h\to{0}}\frac{f(0+h)f(0)}{h}[/tex] 


#5
Nov1404, 07:15 PM

P: 35

yeh i got that to work, now how do I show that it's not a local min,max or inflection. Would I look at the second derivative? If that's not defined it's not anything?



#6
Nov1504, 07:12 AM

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It remains to be shown that f(0) is not a local maximum/minimum. This should be fairly easy to show.. Use, for example, the following definition of local maximum: We say that a function f has a local maximum at [tex]x_{0}[/tex], iff there exists a [tex]\delta>0[/tex] so that for all [tex]x\in{D}(x_{0},\delta),f(x)\leq{f}(x_{0})[/tex] I've assumed that the x's in the open [tex]\delta[/tex]disk are in the domain of f, as is the case in your problem. Note that this definition makes no assumption of differentiability or continuity of f. 


#7
Nov1604, 08:01 PM

P: 35

ok i finished that part of the question.( this is a 4 part question)
I can't figure out these 2 parts. Any tips would be fantastic f(x)=x^2*sin(1/x) 1.let g(x)=2x^2 +f(x) (f from the first question i asked) Show g has a global minimum at x=0 but g'(x) changes sign infinitely often on both (0,e) and (e,0) for any e>0. For this question I can easily show 0 is a critical point. But when I show it's a minimum is what's difficult, when I differentiate twice I cannot see that f''(0)>0 2. Let h(x)=x+2f(x). Show h'(0)>0, but h is not monotone increasing on any interval that includes 0. I know how to show h'(0)>0 but have no idea how to show it's monotone increasing. Again any help would be fantastic thanks in advance 


#8
Nov1704, 07:40 AM

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Show that for 1., g(x)>=x^2 for ALL x.
How can that help you in showing that x=0 must be a global minimum? 


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