I'm looking for a local minimum point that doesn't show up when....

In summary, the conversation discusses finding a local minimum point using the derivative and techniques available. The function in question is arctan(x)-ln(x)/6-x/3 and the discussion also mentions the use of parentheses in equations. It is pointed out that there can only be one real zero of the derivative and complex zeros will not add a local extreme point on the real line. The conversation ends with the clarification that there are actually three real zeros of the derivative, but one is at a negative x value where the function is not defined. The original poster made a mistake, but eventually found the minimum point.
  • #1
Wi_N
119
8
Member warned that the template must be filled in with a problem statement and the work shown

Homework Statement


I set the derivative to 0. i get 1 real root a max point and 2 imaginary. How can i find that local min?

what techniques are available?
 
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  • #2
Wi_N said:

Homework Statement


I set the derivative to 0. i get 1 real root a max point and 2 imaginary. How can i find that local min?

what techniques are available?
How do you know that the critical point is maximum? What is the function you want to find the local minimum of?
 
  • #3
ehild said:
How do you know that the critical point is maximum? What is the function you want to find the local minimum of?

i plotted the graph. there is a min point and a max point. but only the max point is attainable through der=0

arctanx -ln(x)/6 - x/3

max point at x=1 but the min x=0.28 can't be found using der.
 
  • #4
Wi_N said:
i plotted the graph. there is a min point and a max point. but only the max point is attainable through der=0

arctanx -ln(x)/6 - x/3

max point at x=1 but the min x=0.28 can't be found using der.
How did you get the other roots? Show your work.
 
  • #5
ehild said:
How did you get the other roots? Show your work.

thank you for your help. i must be going crazy or blind but i solved it.
 
  • #6
Please use parentheses in your equations to make them clear. They do not cost anything. ;>)

Note: Since arctan is a strictly increasing function, you might get a simpler problem by ignoring the arctan. It depends on whether the problem is ##atan( -ln(x)/6 - x/3 )## or ##atan( -ln(x)/6 ) - x/3 ##.

CORRECTION: I see that the function is atan(x)-ln(x)/6-x/3. Maybe that should have been obvious to me.
 
Last edited:
  • #7
Wi_N said:
thank you for your help. i must be going crazy or blind but i solved it.
It would be useful for other people if you showed the way of solution
 
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  • #8
If there is only one real zero of the derivative (as stated in post #1), there can not be a local max and a local min (as stated in post #3). Complex derivative zeros will not add a local extreme point on the real line.
 
  • #9
FactChecker said:
If there is only one real zero of the derivative (as stated in post #1), there can not be a local max and a local min (as stated in post #3). Complex derivative zeros will not add a local extreme point on the real line.
There are three real zeros of the derivative, but one is at negative x where the function is not defined. The OP made some mistake, and he found it an got the minimum, see Post #5
 
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  • #10
ehild said:
There are three real zeros of the derivative, but one is at negative x where the function is not defined. The OP made some mistake, and he found it an got the minimum, see Post #5
Right. I just wanted to point out that there was a mistake that was clear just from basic principles.
 

1. What is a local minimum point?

A local minimum point is a point on a graph where the function reaches the lowest value within a specific interval. It is lower than all the other points immediately surrounding it, but it may not be the overall lowest point on the entire graph.

2. How do I find a local minimum point?

To find a local minimum point, you must first identify the interval where you suspect the local minimum point exists. Then, you can use a variety of methods such as taking the derivative and setting it equal to zero, using the first derivative test, or graphing the function to visually locate the lowest point within the interval.

3. What does it mean when a local minimum point doesn't show up?

If a local minimum point does not show up on a graph or in a function, it means that there is no point within the specified interval that is lower than all the other points. This could be due to a variety of reasons, such as the function being constant or the interval being too large.

4. Can a local minimum point change?

Yes, a local minimum point can change if the function or the interval is changed. For example, if the function is shifted up or down, the local minimum point will also shift. Similarly, if the interval is expanded or contracted, the local minimum point may also change.

5. How is a local minimum point different from a global minimum point?

A local minimum point is a point on a graph where the function reaches the lowest value within a specific interval. A global minimum point is the overall lowest point on the entire graph. In other words, a local minimum point is only guaranteed to be the lowest point within a certain interval, while a global minimum point is the lowest point on the entire graph.

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