Solve f'(x) = 0: Why is There No Solution?

  • Thread starter nesan
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In summary, the conversation discusses solving the equation f'(x) = 0 for the function f(x) = (ln x + 2x)^(1/3). The attempt at a solution uses Wolfram Alpha to find a solution of x = -1/2, but the textbook states that there is no solution. It is then pointed out that x must be positive for the logarithm to be defined, and that Wolfram Alpha may use a different definition of the logarithm involving complex numbers. This serves as a caution about relying solely on computer software for solutions.
  • #1
nesan
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Homework Statement



http://www.wolframalpha.com/input/?i=((1+/+x)+++2)+/+(3(2x+++lnx))^(2/3)+=+0



Solve the equation f'(x) = 0

f(x) = (ln x + 2x)^(1/3)


The Attempt at a Solution



As you can see, for the function

((1 / x) + 2) / (3(2x + lnx))^(2/3) = 0

I can get a value for x which is - 1 / 2

But my textbook says "no solution"

Why is that?
 
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  • #3
Mark44 said:
x must be positive in order for ln(x) to be defined.

Oh wow, I totally missed that. Thank you very much. :)
 
  • #4
That does not mean that wolfram alpha is wrong however. Wolfram alpha uses a definition of the logarithm which is also defined for negative numbers. This definition uses complex numbers.

However, you likely did not encounter complex numbers and complex logarithms yet, so in your problem x=-1/2 is not a valid solution as the logarithm is not well-defined.

This is a bit the danger of using computer software. Their solutions are not wrong, but they can be different solutions from what you want.
 

1. Why is it important to solve f'(x) = 0?

Solving f'(x) = 0 allows us to find the critical points of a function, which are points where the slope of the function is either zero or undefined. These points are important in determining the behavior of the function and can help us find the maximum and minimum values of the function.

2. Can there be more than one solution to f'(x) = 0?

Yes, there can be more than one solution to f'(x) = 0. This means that there can be multiple critical points on the function where the slope is either zero or undefined.

3. What does it mean if there is no solution to f'(x) = 0?

If there is no solution to f'(x) = 0, it means that there are no critical points on the function. This could indicate that the function is either constantly increasing or decreasing, or that it has no maximum or minimum values.

4. How can we use the solution to f'(x) = 0 to graph a function?

The solutions to f'(x) = 0 can be used to find the x-coordinates of the critical points on the function. These points can then be plotted on a graph to help visualize the behavior of the function and determine its maximum and minimum values.

5. Are there any other methods for finding critical points besides solving f'(x) = 0?

Yes, there are other methods such as using the first and second derivative tests or using technology such as graphing calculators. These methods can also help us find the critical points of a function and determine its behavior.

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