       Re: Integral question 3

• To: mathgroup at smc.vnet.net
• Subject: [mg73716] Re: Integral question 3
• From: "dimitris" <dimmechan at yahoo.com>
• Date: Sun, 25 Feb 2007 04:40:07 -0500 (EST)
• References: <eropmh\$9cb\$1@smc.vnet.net>

```Hello again.

A trick to obtain your "hand" result:

In:=
Integrate[1/Sqrt[EulerGamma^2 - c*s^2], s]
% /. EulerGamma -> v
% /. {v -> 4, c -> 2, s -> 1}
N[%]

Out=
ArcSin[(Sqrt[c]*s)/EulerGamma]/Sqrt[c]

Out=
ArcSin[(Sqrt[c]*s)/v]/Sqrt[c]

Out=
ArcSin[1/(2*Sqrt)]/Sqrt

Out=
0=2E2555251438123125

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 (v4,
c=2)
>
> Integrate[1/Sqrt[4^2 - 2*s^2], s]
>
>
> ArcSin[s/(2*Sqrt)]/Sqrt
>
> 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.
>