Finding x in cos(x)=exp(-2x): Analytical Solutions?

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Discussion Overview

The discussion revolves around the equation cos(x) = exp(-2x) and the search for analytical solutions for x. Participants explore the nature of the equation, its transcendental functions, and the potential for finding solutions through various methods, including graphical analysis.

Discussion Character

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

Main Points Raised

  • One participant inquires about the possibility of finding analytical solutions for x in the equation cos(x) = exp(-2x), noting that x=0 is an obvious solution and that numerical methods yield x=1.5232.
  • Another participant asserts that solving the equation involves two different transcendental functions, suggesting that there is generally no simple algebraic method to find solutions.
  • A participant expresses appreciation for the confirmation that the problem is indeed complex and not straightforward.
  • Graphical analysis is proposed as a method to understand the behavior of the functions, with one participant noting that the graphs intersect infinitely often, with solutions approaching the zeroes of cos(x) as x increases.
  • Another participant shares their experience of plotting the functions in MATLAB, observing that the second solution is very close to pi/2, which aligns with the graphical insights discussed.

Areas of Agreement / Disagreement

Participants generally agree on the complexity of the equation and the limitations of finding analytical solutions. However, there is no consensus on a definitive method for solving the equation, and multiple views on the behavior of the solutions are presented.

Contextual Notes

The discussion highlights the challenges of solving equations involving transcendental functions and the reliance on numerical and graphical methods, without resolving the underlying mathematical complexities.

Swest
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Hello all

Can anybody see a way to analytically find x in the expression:

cos(x) = exp(-2x)

By inspection x=0 is obvious, and numerically we find x=1.5232 is also a solution, but is there a way to find these values by rearranging the above expression? It's one of those that looks simple but isn't, any ideas?

Many thanks
 
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That is not "simple" algebra. What you are attempting to do is solve an equation involving two different transcendental functions. In general there is no simple way to do that.
 
Thanks, I suspected as much. Always nice to be reassured that one isn't missing something obvious.
 
graphing them is instructive as it shows the graphs meet infinitely often, at points that become closer and closer to the zeroes of cos(x),

i.e. the solutions approach closer and closer to the integral multiples of pi/2.
 
mathwonk said:
graphing them is instructive as it shows the graphs meet infinitely often, at points that become closer and closer to the zeroes of cos(x),

i.e. the solutions approach closer and closer to the integral multiples of pi/2.

that make sense seeing as for large x the exponential is vanishing.
 
Yes, I did a quick MATLAB plot of the two and saw the trend that you describe, which again can be surmised from inspection, indeed the 2nd solution is very close to pi/2.
 

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