Local Minimum of x^4-9x^3+9x^2+5x-4 - Where am I Wrong?

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Homework Help Overview

The problem involves finding the local minimum of the polynomial function x^4-9x^3+9x^2+5x-4. The original poster expresses confusion regarding the distinction between local and absolute minima, particularly in relation to the values x=-0.21 and x=5.96.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the definitions of local and absolute minima, questioning whether all absolute minima are also local minima. There is an exploration of the implications of these definitions in the context of the problem.

Discussion Status

Participants have engaged in clarifying the relationship between local and absolute minima. Some have confirmed that the absolute minimum is indeed a local minimum, while others are still grappling with the implications of this understanding. The discussion is ongoing with multiple interpretations being explored.

Contextual Notes

The original poster appears to be operating under the assumption that there can be a distinction between local and absolute minima, which is being questioned by other participants. There is a focus on the specific values provided in the problem and their classifications.

koudai8
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The question asks for local minimum of x^4-9x^3+9x^2+5x-4.

The answer was x=-0.21 and 5.96.

I thought 5.96 is the absolute minimum, since it gives the loweset y value on the open interval. Where am I wrong.

Thanks.
 
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An absolute minimum is a local minimum; if it's a minimum across the entire interval, it's obviously a minimum locally.
 
ideasrule said:
An absolute minimum is a local minimum; if it's a minimum across the entire interval, it's obviously a minimum locally.

So all absolute are also locals? Never knew that. Thanks.
 
You aren't wrong about x=5.96 being the absolute minimum. But the problem is asking for local minima. There's more than one.
 
Dick said:
You aren't wrong about x=5.96 being the absolute minimum. But the problem is asking for local minima. There's more than one.

I just thought the other value is the absolute minimum, not the local minimum, so I didn't include it as the local minimum. So, the absolute is ALWAYS also a local one?
 
koudai8 said:
I just thought the other value is the absolute minimum, not the local minimum, so I didn't include it as the local minimum. So, the absolute is ALWAYS also a local one?

Sure it is. ideasrule has it correct.
 
Dick said:
Sure it is. ideasrule has it correct.

That's all I need. Thank you both.
 

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