Solving this logarithmic equation analytically

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  • Thread starter Thread starter Mr Davis 97
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Discussion Overview

The discussion centers around the analytical solution of the logarithmic equation ##3x + \log_5x = 378##. Participants explore various methods for solving this transcendental equation, including numerical approaches, transformations, and graphical methods.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the equation can be solved by trying small powers of ##5##.
  • Another participant proposes using ##3x=378## to get an initial guess for ##x##, leading to a solution of ##x=125##.
  • It is noted that such equations typically do not yield nice solutions, and numerical methods are often required.
  • A participant mentions the Lambert W function as a potential method for solving the equation, although they express uncertainty about understanding this approach.
  • Another contribution discusses finding a transformation to obtain a closed-form solution, emphasizing the need for a classification of the transformation.
  • One participant shares a graphical method to find the solution, indicating that the intersection of the plotted functions reveals the solution at ##(125, 378)##.
  • A later reply clarifies that the graphical method is not an analytical solution, which requires algebraic means.

Areas of Agreement / Disagreement

Participants express differing views on the methods of solving the equation, with some advocating for numerical or graphical approaches while others suggest analytical transformations or the Lambert W function. No consensus is reached on a singular method for an analytical solution.

Contextual Notes

Participants acknowledge the limitations of their proposed methods, including the complexity of transformations and the general nature of transcendental equations that may not yield simple solutions.

Mr Davis 97
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I have the equation ##3x + \log_5x = 378##.

Is there an analytical way to solve for x? Or for this equation are we forced to just try possible values, such as powers of 5?
 
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It is a transcendental equation, but in this case you can indeed find its solution by trying some small powers of ##5##.
 
To get a good first guess use 3x=378 or x=126. You will quickly get to the solution (x=125).
 
Note that in general such equations do not have very nice solutions. But it appears you got lucky here. Normally, you would need to resort to numerical answers.
 
You can try using Lambert W function or https://www.math.ucdavis.edu/~thomases/W11_16C1_lec_3_11_11.pdf to solve this equation. Unfortunately, I don't quite fully understand these methods (and I hope my test won't have these..I still don't quite understand these methods even after trying to solve online algebra practice tests..the methods proposed above are easy but I don't think they'll come in handy on a test) but I've seen such problems being solved with the help of Lambert W function. So, if you are better at it you can try doing it. And here is an equation similar to yours.
 
Last edited:
Hey Mr Davis 97.

Aside from a numerical solution, you will have to find a transformation so that the transformation allows one to get a solution in some basis (like integers, rationals, or some function of other quantities).

For this problem you will have to find a transformation u(x) to take x to u (and that preserves the inequality) where the transformation has a classification so that the solution is in closed form.

You could start by noting that log_a(x) + log_a(y) = log_a(xy) and mucking around with that from here on in.
 
Hello!
I know the only one analytical way to solve it - with plots. Here is attached the plot of two parts of equation( you can do the same one with your favorite graphics builder( i prefer wolfram ar https://handmadewritings.com). On this plot you can easily see the only point that belongs to these two functions at the same time and it is (125, 378). That's how you can get your answer 125!
2f08e41e8ab144e2a190d320347382ed.png
 
BaileyBelmont said:
I know the only one analytical way to solve it - with plots.
That's a graphical solution, not an analytic solution, which means finding a solution by algebraic means.
 

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