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MathGroup Archive 2005

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Re: Simplifying Log to ArcCos Expressions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56874] Re: Simplifying Log to ArcCos Expressions
  • From: "Scout" <user at domain.com>
  • Date: Mon, 9 May 2005 01:45:56 -0400 (EDT)
  • References: <d5kapv$299$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi David,
I have just tried to solve the integral you posted
and on Math.4 it has given to me:

In[1]:=    expr1 = (k/r^2)*(1/Sqrt[1 - k^2/r^2]);
               expr2 = Integrate[expr1, r]

Out[1]:=    -ArcSin[k / r]

I don't know your problem about this on Math.5.

    ~Scout~

> Dear MathGroup,
>
> I want to integrate the following expression and get a simple answer.
>
> expr1 = (k/r^2)*(1/Sqrt[1 - k^2/r^2])
>
> The answer is actually quite simple: ArcCos[k/r] + constant. But what a 
> lot of work for me to get it! Perhaps someone can show a simpler path. 
> (I'm working with Version 5.0.1.)
>
> expr2 = Integrate[expr1, r]
> -((Sqrt[k^2 - r^2]*Log[(2*(k + Sqrt[k^2 - r^2]))/
>      r])/(Sqrt[1 - k^2/r^2]*r))
>
> Then I have to do all the following simplification steps...
>
> expr2[[{2, 3, 4}]]
> Numerator[%]/(Denominator[%] /. Sqrt[a_]*(b_) :>
>    Sqrt[Distribute[a*b^2]])
> % /. (a_)^(1/2)/(b_)^2^(-1) -> (a/b)^(1/2)
> Simplify[%, r >= k]
> Expand[%*FunctionExpand[expr2[[{1, 5}]]]]
> expr3 = %[[2]]
>
> expr3
> MapAt[Distribute, %, {{2, 1}}]
> % /. Sqrt[a_]/(b_) :> Sqrt[Distribute[a/b^2]]
> % /. r -> k/z
> % /. Log[(z_) + Sqrt[(z_)^2 - 1]] -> I*ArcCos[z]
> % /. z -> k/r
>
> Thanks in advance for a more direct path.


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