What math do I need to solve this: 0.5x^2 - 2x + lnx = -1.687?

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Discussion Overview

The discussion revolves around the mathematical methods required to solve the equation 0.5x^2 - 2x + lnx = -1.687. Participants explore the nature of the equation, its solvability, and potential approaches for finding solutions, including algebraic and numerical methods.

Discussion Character

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

Main Points Raised

  • One participant seeks clarification on the terminology and methods needed to solve the equation algebraically.
  • Another participant asserts that the equation is not solvable algebraically in terms of standard functions, suggesting the Lambert-W function as a possible avenue.
  • A different participant emphasizes that even simpler related equations do not yield nice algebraic solutions, indicating that solutions may involve functions that define the solution implicitly.
  • One participant expresses uncertainty about missing any obvious methods, such as the use of complex numbers, but acknowledges the difficulty of finding an algebraic solution.
  • Participants mention that numerical methods, including graphing, can provide approximations to the solution, with one participant stating an approximate solution of 0.351 derived from graphing.
  • Another participant clarifies that the value provided is an approximation and reiterates that the equation is not solvable algebraically, emphasizing the use of numerical means for approximations.

Areas of Agreement / Disagreement

Participants generally agree that the equation cannot be solved algebraically in a straightforward manner and that numerical methods are necessary for approximating solutions. However, there is no consensus on the exact nature of the solutions or the applicability of the Lambert-W function.

Contextual Notes

Some assumptions about the nature of the functions involved and the limitations of algebraic solutions are present, but these remain unresolved within the discussion.

tmoney120
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TL;DR
How do you solve an equation that has both a quadtratic and logarithmic term?
I am looking for information on this type of equation but I don't know the terminology so I could not find anything for these types of equations:

0.5x^2 -2x + lnx = -1.687

What math do I need to solve something like this algebraically?
 
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Such things are not really solvable algebraically. I.e. you won't be able to get an answer in terms of standard functions.

You might be able to express in answer in terms of the Lambert-W function though. See https://en.wikipedia.org/wiki/Lambert_W_function
 
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You won't get a nice algebraic solution. Even for a much simpler case, x+ln(x)=c there is no such thing (with exceptions, of course). There are solutions with functions that basically say "f(x) is the solution. Here is your solution: f(x)" and there might be solutions that involve some limit processes.
 
Thanks! I thought so, but wanted to see if I missed something obvious, maybe complex numbers or something, but anyway, thanks and see you around the forums!

The answer is 0.351 by the way, and I got that by graphing the function.
 
tmoney120 said:
The answer is 0.351 by the way, and I got that by graphing the function.
No, that's an approximation to the answer. The other people responding to your question said that the problem isn't solvable algebraically (except possibly by the Lambert W function), but one can use numerical means, including looking at a graph, to get a reasonably close approximation.
Wolframalpha gives the solution as .351307, which is again an approximation, and not the exact solution.
 
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