Integration Technique

  • Thread starter Sparky_
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  • #1
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Main Question or Discussion Point

Greetings,

I know [tex] \int \frac {1} {{\sqrt{1-x^2}} dx[/tex] is arcsine(x)

My question is can [tex] \int \frac {1} {\sqrt{1-x^2}} dx[/tex]
be solved by some technique (parts, substitution...) and the answer be in terms of x and NOT an expression of arcsine?

Meaning, I would like the solution to the integral in terms of x and possibly other functions (ln for example) but not in terms of trig functions.

If so, can you show me?

Thanks
Sparky_
 
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Answers and Replies

  • #2
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It can be done by an appropriate trig substitution ie. x=sin(y), but the answer will end up identically. I'm fairly certain that arcsin(x) is the only answer you will ever get though.
 
  • #3
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Antiderivatives are unique up to a constant. Ie, all antiderivatives of that will be arcsine(x)+C.
 
  • #4
Hurkyl
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The only two other (sane) ways I can think of to write that antiderivative is as a power series, or in terms of a complex logarithm. But any of those ways will simply be a different way of writing arcsin x.
 
  • #5
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One could certainly express arcsin in terms of logs using Euler's formula...
 
  • #6
Gib Z
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One could certainly express arcsin in terms of logs using Euler's formula...
Yea, Hurkyl already mentioned that.

...or in terms of a complex logarithm
So yes I think the OP is happy,

possibly other functions (ln for example)[\QUOTE]
 
  • #7
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What I am wanting is an expression for arcsine that uses other functions (hopefully LN and powers of x) but I am hoping to work in the real domain.

Gib Z help me quite a bit with a derivation regarding arcsine.

I was hoping some clever integration could come up with another expression equal to arcsin by integrating the derivative of arcsine.

Any thoughts?

Thanks
Sparky_
 
  • #8
HallsofIvy
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If trig substitution, which just about everyone mentioned, is not a "clever integration", I don't know what is!
 
  • #9
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I agree trig substitution is clever, I was hoping for a solution that doesn't include trig functions.

-Sparky_
 
  • #10
Gib Z
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I remember what sparky wanted from his previous thread that i tried to help him in. He had [tex]\arcsin x = -i \ln ( ix \pm \sqrt{1-x^2})[/tex] as he should, but could not prove why the expressions were equal.
 
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  • #11
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Correct Gib_Z and you did help me prove it - you helped me get the true solution and in that I ended up with some other solutions that I assumed involved the other quadrants.

Using that expression (arcsin = -iln(...) has become very involved due to working in the complex domain.

I'm now searching for a similar type of solution in the real domain only. (I suppose this rules out Euler's)

-Sparky_
 
  • #12
Gib Z
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Well then the best answer I can give you is the Taylor Series for Arcsin x, which If i remember correctly is this:

[tex]\arcsin x = \sum^{\infty}_{n=0} \frac{(2n)!}{4^n (n!)^2 (2n+1)} x^{2n+1}[/tex]
 
  • #13
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Thank you all for your help.

Sparky_
 

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