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Re: Beginner--How to get a positive solution from Solve Command

• To: mathgroup at smc.vnet.net
• Subject: [mg67807] Re: Beginner--How to get a positive solution from Solve Command
• From: dimmechan at yahoo.com
• Date: Sat, 8 Jul 2006 04:56:25 -0400 (EDT)
• References: <e8lgq7$ras$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Î?/Î? abdou.oumaima at hotmail.com έγÏ?αÏ?ε:
> Hello mathgroup,
>
> I'm trying to solve and equation in a limit case, so I've to use Taylor series.
>
> In[1]: Normal[Series[1 - c  ArcTanh[ξ]/ξ, {ξ, 0, 2}]]
> Out[1]: 1-c-c (ξ^2/3)
>
> and
>
> In[2]: Solve[%==0,ξ]
> Out[2]: {{ξ->- (3(1-c)/ξ)^1/2},{ξ->(3(1-c)/ξ)^1/2}}
>
> I need just the positive solution. How to force Solve to give me the positive value please.
>
> Any Help please.
>
> Thank you
>
I am not a specialist, but the following command seems to do what you
want:

ln[1]: DeleteCases[ξ /. Solve[Normal[Series[1 - c ArcTanh[ξ]/ξ, {ξ,
           0, 2}]] == 0, ξ], -a_]
Out[1]: \!\({\(\@3\ \@\(1 - c\)\)\/\@c}\)

I hope to help you.