Solving Integral with u=tan(x/2): Hints & Suggestions

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Discussion Overview

The discussion revolves around solving the integral of the form integral(dx / (3cosx - 4sinx)). Participants explore the substitution u = tan(x/2) as a method to simplify the integral, discussing various approaches and hints related to trigonometric identities and integration techniques.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant requests ideas for solving the integral and mentions difficulty with the substitution u = tan(x/2).
  • Another participant suggests using the identity for tan(x) to relate it to sin(x) and cos(x), hinting at a possible partial fractions approach.
  • A third participant emphasizes the need for half-angle or double-angle formulas for sin(x) and cos(x) to facilitate the integration process.
  • One participant provides a detailed substitution method, including expressions for dx, sin(x), and cos(x) in terms of u.
  • Several participants engage in clarifying the terminology used in the discussion, with one correcting another's reference to a "warning" as a misunderstanding of the post numbering.
  • Another participant expresses gratitude for the clarification and indicates understanding of the substitution method.

Areas of Agreement / Disagreement

Participants generally agree on the utility of the substitution u = tan(x/2) and the need for trigonometric identities, but there is no consensus on the best approach to proceed with the integral. Some participants express confusion about the details of the substitution.

Contextual Notes

Some participants mention specific trigonometric identities and their applications, but the discussion does not resolve the integral itself or provide a definitive method for solving it.

chimera
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could you please give me any idea to solve the problem below;

integral( dx / (3cosx-4sinx) )


and given a hint to make a subtitution u=tan(x/2), I've tried to write cosx and sinx in the form of cos (x/2) and sin(x/2), but it's seems like I'm not going anywhere, any suggestions?
 
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[tex]\int (\frac{dx}{3cosx - 4sinx})[/tex]

Does this help?:

[tex]\int tanxdx = \int \frac{sinx}{cosx}dx[/tex]

It sounds like a partial fractions to me.

The Bob (2004 ©)
 
Yes, you have to use the half angle formulas (or is it double angle formula?) for sin x and cos x.

[tex]\cos^2{x} = \frac {1 + \cos{2x}}{2} \ \ \mbox{and} \ \ \sin^2{x} = \frac {1-\cos{2x}}{2}[/tex]
 
It looks really ugly.

[tex]I=:\int \frac{dx}{3\cos x-4\sin x}[/tex](1)

Make the substitution:

[tex]x=2\arctan u (<=> u=\tan\frac{x}{2})[/tex] (2)

,under which simple trigonometry and differentiation will show that

[tex]dx=\frac{2 du}{1+u^{2}}[/tex] (3)

[tex]\sin x= \frac{2u}{1+u^{2}}[/tex] (4)

[tex]\cos x=\frac{1-u^{2}}{1+u^{2}}[/tex] (5)

Can u continue from here...?

Daniel.
 
thanks, i got it =)
 
Dextercioby, I don't get what u wrote. in the 4th warning. the (2) would u tell me. thanx
 
One-D: It's 4'th POST, not WARNING!
Daniel made a very common and useful change of variables.
That's all there is to it.
 
Incidentally i have 4 warnings...:smile: :rolleyes:

Daniel.
 
dextercioby said:
Incidentally i have 4 warnings...:smile: :rolleyes:

Daniel.
I already knew you were a good and inoffensive boy..:wink:
 
  • #10
Thanks for the trust.Marlon feels the same way,though i don't remember any warning gotten from the clashes we've had...:wink:

Daniel.
 
  • #11
thanx. know i understand. it's only a simple subs. thanks anyway.
 

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