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MathGroup Archive 2007

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Re: Integral question 4

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73720] Re: Integral question 4
  • From: "dimitris" <dimmechan at yahoo.com>
  • Date: Sun, 25 Feb 2007 04:42:16 -0500 (EST)
  • References: <eropmh$9cb$1@smc.vnet.net>

Hello again (I promise it is the last message!)

Look the thread here:

http://groups.google.gr/group/comp.soft-sys.math.mathematica/browse_thread/thread/e25123d7108d475a/8dcbcacb39abd299?lnk=gst&q=Integrate&rnum=734&hl=el#8dcbcacb39abd299

Dimitris


=CF/=C7 Tulga Ersal =DD=E3=F1=E1=F8=E5:
> Dear Mathematica users,
>
> Let's consider the integral
>
> Integrate[1/Sqrt[v^2 - c*s^2], s]
>
> where v and c are positive reals. When I calculate the integral by hand, 
I get
>
> ArcSin[(Sqrt[c]*s)/v]/Sqrt[c]
>
> However, if I evaluate the integral in Mathematica, I get
>
> (I*Log[(-2*I)*Sqrt[c]*s + 2*Sqrt[-(c*s^2) + v^2]])/Sqrt[c]
>
> which not only does not look like what I have found by hand, but
> also, for v=4, c=2, s=1, for example, gives
>
> 0.255525 + 1.47039 i
>
> as opposed to just 0.255525.
>
> If you evaluate the integral in Mathematica with the said values (v=4, 
c=2)
>
> Integrate[1/Sqrt[4^2 - 2*s^2], s]
>
> you get the answer
>
> ArcSin[s/(2*Sqrt[2])]/Sqrt[2]
>
> which agrees with what I have found by hand.
>
> My question is: How can I get what I found by hand using Mathematica
> without having to assign values to v and c before evaluating the
> integral? I tried adding the assumptions v>0, c>0, but it didn't help.
>
> I'd appreciate your help.
>
> Thanks,
> Tulga



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