Help with extracting alpha from: -alpha-asin(sin(alpha)(r/l)

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Discussion Overview

The discussion revolves around extracting the variable \(\alpha\) from a complex equation involving the Law of Cosines and trigonometric identities. The context includes mathematical reasoning and potential methods for solving the equation, with a focus on the implications of the terms involved.

Discussion Character

  • Mathematical reasoning
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents an equation involving \(\alpha\) and seeks assistance in solving for it, treating other variables as constants.
  • Another participant points out a potential issue with the brackets in the arcsine function and notes that \(\text{asin}(\sin(x)) = x\).
  • A participant expresses concern about the term \((r/l)\) in the equation and questions the validity of \(\text{asin}(\sin(x)(r/l))\) equating to \((r/l)x\).
  • Another participant suggests that the equation cannot be solved for \(\alpha\) in a straightforward manner and proposes that a clever setup, approximation, or numerical method may be necessary.
  • It is noted that the arcsine function may be undefined for certain values of \(\alpha\), depending on the relationship between \(r\) and \(l\), specifically that \(|\sin(\alpha)| < l/r\).

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of extracting \(\alpha\) from the equation, with some suggesting that it may not be possible without additional methods or approximations. There is no consensus on a definitive approach to solving the equation.

Contextual Notes

The discussion highlights potential limitations related to the undefined nature of the arcsine function for certain values of \(\alpha\) and the need for careful consideration of the terms involved in the equation.

dbarger1225
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I am using the Law of Cosines to extract and angle I need from a shaft that is turning on an air engine by piston oscillation and I am having a brain fart on if there are any identities I am missing that can help me pull '\alpha' from the right side of my equation:

acos(((h-pl)^{2}-r^{2}-l^{2})/(-2*r*l))-\pi=-\alpha-asin(sin(\alpha)(r/l))

I would like to solve the entire equation for \alpha and the other variables can be treated like constants.

Thanks for the help or attempts in advance.
 
Last edited:
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well - you didn't close the brackets in the arcsine, so it's hard to tell - but asin(sin(x))=x
 
I fixed the bracket issue, I apologize about that.

I am concerned with the (r/l) term inside of the equation. I understand that asin(sin(x))=x .. but I'm certain that asin(sin(x)(r/l)) --DNE-- (r/l)x
 
Oh I think I see, it wasn't clear to me if the r/l was multiplied with the alpha or the sine-alpha, or with the arcsine.
This what you mean?

\sin^{-1}( \frac{r}{l}\sin\alpha )

... afaik you can't extract alpha from this sort of equation - you need to be cleverer in the setup or use an approximation (or use a numerical method).

Equation is of form: x+sin-1(A.sin(x))=b ... solve for x. argh. (assuming the RHS is all constants)

Note: the arcsin will be undefined for some alpha, depending on the value of rl - you want |sin(α)| < l/r (something...)

Where did you start from?
 
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