Solving Linear First Order D.E. y' - y*tan[x] = 2sin[x]

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Discussion Overview

The discussion revolves around solving the linear first-order differential equation y' - y*tan[x] = 2sin[x]. Participants are examining different approaches to arrive at the correct solution and reconciling discrepancies between their results and the textbook answer.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents their solution as y = 2ln(sec[x]) / sec[x] - c/sec[x] and questions how to arrive at the textbook answer y = (c - cos[x]) * sec[x].
  • Another participant asserts that the two solutions are not equivalent due to the presence of the natural logarithm, suggesting that at least one solution must be incorrect.
  • A third participant claims the textbook answer is correct and points out that integrating will yield a ln(y) along with ln(sec(x)).
  • This participant also describes a method for solving the equation by separating the homogeneous and non-homogeneous parts and suggests using variation of parameters or guessing a simple solution.
  • A later reply expresses gratitude for the assistance and indicates that they have resolved their confusion.

Areas of Agreement / Disagreement

Participants do not reach consensus; there are competing views on the correctness of the solutions presented, with some asserting the textbook answer is valid while others believe the initial solution is also plausible.

Contextual Notes

There are unresolved assumptions regarding the integration process and the equivalence of the two proposed solutions, as well as the implications of the natural logarithm in the context of the differential equation.

splitendz
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Solve y’ – y*tan[x] = 2sin[x].

I keep arriving at the answer: y = 2ln(sec[x]) / sec[x] - c/sec[x]
for this question.

According to my textbook the correct answer is: y = (c - cos[x]) * sec[x]. Can anyone explain how to obtain this answer?

Thanks :)
 
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Show us your work.Because of that natural logarithm,the two answers are not equivalent (equal for some value of the integration constant),therefore at least one of them is wrong.

Daniel.
 
The text answer is correct. I suspect you overlooked the fact that integrating will give you a ln(y) as well as "ln(sec(x))".

Since this is a linear equation, you can do the homogeneous and non-homogeneous parts separately. The associated homogeneous equation is just y'- y tan x= 0 or
y'= y tan x so dy/y= tan x dx= (sin x dx)/cos x.
To find a solution to the non-homogeneous part you could use "variation of parameters": try a solutution of the form y(x)= u(x) sec(x) (I'm giving away part of the answer there!), differentiate and plug into the equation to find a simple equation for u(x).
Or just guess a simple solution!
 
Ah right. Thanks a lot for your help HallsofIvy! I've got it now.
 

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