Usage of variable in integration

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    Integration Variable
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Discussion Overview

The discussion revolves around the usage of variables in integration, particularly in the context of differential equations. Participants explore the implications of changing variables in integrals and the independence of those variables.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions whether the variables ##f## and ##\theta## should be considered independent when performing the change of variable ##f=\sin(\theta)\sin(u)##.
  • Another participant asserts that in the integral, ##\theta## is treated as a constant, leading to the conclusion that the change of variable formula is ##df = \sin\theta\ \cos u\,du##.
  • A different participant raises a question about the dependency of ##\theta## on ##f## and what implications this has for ##d\theta##.
  • One participant presents an example where they integrate the expression and derive a relationship, but they note that it does not satisfy the original differential equation, prompting a query about the reason for this discrepancy.
  • A later reply reiterates the question about the independence of ##f## and ##\theta## and emphasizes that if ##f## is a function of ##\theta##, then generally, ##\frac{df}{d\theta}## would not be zero.

Areas of Agreement / Disagreement

Participants express differing views on the independence of the variables involved in the integration process. There is no consensus on whether ##f## and ##\theta## should be treated as independent or dependent variables.

Contextual Notes

There are unresolved assumptions regarding the relationships between the variables and the implications of the change of variable on the integration process. The discussion also highlights the complexity of the differential equation involved.

jk22
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When solving differential equations the following scripture can arise, for example:

$$\int \frac{df}{\sqrt{\sin(\theta)^2-f^2}}$$

If the change of variable ##f=\sin(\theta)\sin(u)##

Is performed, do the letters ##f,\theta## shall be considered independent or is

$$df=\cos(\theta)\sin(u)d\theta+\sin(\theta)\cos(u)du$$ ?
 
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In that integral, ##\theta## is treated as a constant, so the change of variable formula is ##df = \sin\theta\ \cos u\,du##
 
What is the application? does θ depend on f ? Suppose θ is independent of f - then what would you expect to be?
 
I took an example supposing $$\int\frac{df}{\sqrt{\sin(\theta)^2-f^2}}=\frac{1}{\theta}$$

It comes from the differential equation $$\frac{df}{d\theta}=-\frac{\sqrt{\sin(\theta)^2-f^2}}{\theta^2}$$

If we put ##df=\sin\theta\cos udu## we can integrate to ##u=\frac{1}{\theta}\Rightarrow f=\sin\theta\sin(\frac{1}{\theta})##

But it does not satisfy the differential equation. Why is this ?
 
jk22 said:
If the change of variable
##f=\sin(\theta)\sin(u)## Is performed, do the letters ##f,\theta## shall be considered independent or is
$$df=\cos(\theta)\sin(u)d\theta+\sin(\theta)\cos(u)du$$ ?
It would have been helpful to include the differential equation in post #1.
jk22 said:
It comes from the differential equation $$\frac{df}{d\theta}=-\frac{\sqrt{\sin(\theta)^2-f^2}}{\theta^2}$$
If f is a function of ##\theta##, the ##\frac{df}{d\theta}## won't be zero, in general.
 

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