How can I solve inequations with logarithms?

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    Logarithm
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Discussion Overview

The discussion revolves around solving inequations involving logarithmic functions, specifically two cases presented by the original poster. The focus includes theoretical approaches, potential methods for approximate solutions, and the challenges associated with these types of inequalities.

Discussion Character

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

Main Points Raised

  • The original poster presents two inequations involving logarithms and requests suggestions for books on the topic.
  • Some participants express skepticism about the possibility of explicitly solving for x in the given inequations, questioning whether the examples are from a textbook or made up.
  • One participant suggests that a standard approach to solving complicated inequalities is to first solve the corresponding equations to identify critical points where the inequality changes.
  • Another participant mentions that there are no algebraic methods for solving equations where variables appear both inside and outside of logarithmic functions.
  • There are inquiries about approximate methods for solving the inequations, with a suggestion to use numerical methods, specifically Newton's method, which requires specific numerical values for the constants involved.

Areas of Agreement / Disagreement

Participants generally agree on the challenges of solving the inequations explicitly, with multiple views on the feasibility of finding solutions and the appropriateness of numerical methods. The discussion remains unresolved regarding the best approach to take.

Contextual Notes

The discussion highlights limitations in solving the inequations due to the nature of logarithmic functions and the presence of transcendental equations. Specific numerical values for constants are necessary for applying numerical methods.

quacam09
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Hi all,

Do you know how to solve the following inequations?

<br /> \ln \left( {\frac{{x + a}} {{x + b}}} \right) \leq cx + d
<br /> \ln \left( {\frac{{x + a}} {{x + b}}} \right) \geq \frac{{x^3 + cx^2 + dx + e}} {{ux^2 + vx}} <br />

a, b, c, d, e, u, v are constants.
x is a variable.

Can you suggest me a book about solving inequations containing logarithm?
Thanks!
 
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I don't even think it is possible to explicitly solve for x in these two cases. Did you just make these examples up, or you got them from a textbook?
 
quacam09 said:
Hi all,

Do you know how to solve the following inequations?

<br /> \ln \left( {\frac{{x + a}} {{x + b}}} \right) \leq cx + d
<br /> \ln \left( {\frac{{x + a}} {{x + b}}} \right) \geq \frac{{x^3 + cx^2 + dx + e}} {{ux^2 + vx}} <br />

a, b, c, d, e, u, v are constants.
x is a variable.

Can you suggest me a book about solving inequations containing logarithm?
Thanks!
The standard way of solving complicated inequalities is to solve the corresponding equation first, giving values at which ">" changes to "<", then decide which of the area between those are ">" and which "<".

However, as sutupid math said, there are no "algebraic" methods of solving equations in which the variables appear both in and out of the logarithm (or, more generally, any transcendental functions).
 
HallsofIvy said:
The standard way of solving complicated inequalities is to solve the corresponding equation first, giving values at which ">" changes to "<", then decide which of the area between those are ">" and which "<".

However, as sutupid math said, there are no "algebraic" methods of solving equations in which the variables appear both in and out of the logarithm (or, more generally, any transcendental functions).

Thank you for your response. Are there any method to solve approximately it? Can you give me a suggestion?
 
sutupidmath said:
I don't even think it is possible to explicitly solve for x in these two cases. Did you just make these examples up, or you got them from a textbook?

Thank you.
 
quacam09 said:
Thank you for your response. Are there any method to solve approximately it? Can you give me a suggestion?
Sure. Of course, numeric methods will require that you give specific numerical values to a, b, c, and d. I would recommend "Newton's method". To solve
ln(\frac{x+a}{x+b}= ln(x+a)- ln(x+b)= cx+ d,
let f(x)= ln(x+a)- ln(x+b)- cx- d
You want to find x such that f(x)= 0. Choose some starting x_0 and then use the iteration
x_{n+1}= x_n+ \frac{f(x_n}{f&#039;(x_n)}.
 
HallsofIvy said:
Sure. Of course, numeric methods will require that you give specific numerical values to a, b, c, and d. I would recommend "Newton's method". To solve
ln(\frac{x+a}{x+b}= ln(x+a)- ln(x+b)= cx+ d,
let f(x)= ln(x+a)- ln(x+b)- cx- d
You want to find x such that f(x)= 0. Choose some starting x_0 and then use the iteration
x_{n+1}= x_n+ \frac{f(x_n}{f&#039;(x_n)}.
Thank you.
 

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