Solve Inequality x+3^x<4 | Logical & Analytic Ways

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  • Thread starter Thread starter Karim Habashy
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Discussion Overview

The discussion revolves around solving the inequality x + 3^x < 4, exploring both logical and analytic methods. Participants examine various approaches, including trial and error, calculus, and the Lambert W function, while seeking to establish the validity of proposed solutions.

Discussion Character

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

Main Points Raised

  • One participant claims to have found the solution x < 1 using trial and error and seeks a more logical or analytic method.
  • Another participant states that there is no analytic solution for the zeroes of the equation x + 3^x - 4, suggesting numerical or observational methods instead.
  • A participant proposes using proof by contradiction to explore values of x > 1, while another clarifies that this approach does not imply that x < 1 must satisfy the inequality.
  • Calculus is suggested as a method to demonstrate that no values of x > 1 satisfy the inequality.
  • One participant notes that since x + 3^x has a positive gradient everywhere, it can be argued that all x < 1 must satisfy the inequality x + 3^x < 4.
  • Another participant mentions the possibility of using the logarithmic product function as an alternative approach.

Areas of Agreement / Disagreement

Participants express differing views on the validity of various methods for proving the inequality, with no consensus on a single approach. The discussion remains unresolved regarding the best method to establish the solution.

Contextual Notes

Participants acknowledge limitations in their approaches, such as the lack of an analytic solution and the need for careful consideration of assumptions when using proof techniques.

Karim Habashy
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Hi all,

I was trying to solve the Inequality x+3x<4 and i found the solution to be x<1, using trial and error.
Is there another Logical way or analytic one.

Thanks
 
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There is no analytic solution to finding the zeroes of the equation x+3x-4, hence there is no solution other than by numerical or observational means to solve yours (unless you're willing to use the Lambert W function). What you can do however is to prove that x<1 is the only possibility. What if there are other values of x > 1 that work? Can you prove there aren't?
 
Ya, strengthening my answer by contradiction is a good idea (i.e what happens if x>1).
I will also have a look at Lambert W Function.

Thanks
 
Sorry about the late response, I guess I missed this thread in my alerts.

Just to be clear, proving that the equation cannot hold for x>1 is NOT a proof by contradiction. You can't automatically assume that if x>1 does not satisfy the inequality, then x<1 must satisfy it. It doesn't work that way.

I'd suggest calculus for showing that there are no values x>1 that satisfy the equation.
 
You could simply observe that x+3^x=4 at x=1, and argue that since x+3^x has a positive gradient everywhere, all x which satisfy x<1 must satisfy x+3^x<4
 
How do i use calculus ?, the function is monotonically increasing.
 
By stating that the function is monotonically increasing, you have used Calculus. And the fact that it is monotonically increasing leads to Homogeneous Cow's point, that there can only be one point where f(x)= 4.
 
Trial and error is the way to go, unless you want to put the log product function into play.
 

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