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MathGroup Archive 2006

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Re: Possible bug concerning a limit computation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg71308] Re: Possible bug concerning a limit computation
  • From: dh <dh at metrohm.ch>
  • Date: Tue, 14 Nov 2006 05:06:28 -0500 (EST)
  • References: <eish78$nrd$1@smc.vnet.net>

Hi Kostas,
one usually assumes the convention that the square root of a positive 
number is positive. However, could it be, that by the way you calculate 
the square roots, the negative branch cut is taken? If you do this with 
the first and one of the second square roots, you get -9/2.
Daniel

geo_topos wrote:
> Hello everyone,
> I don't know if this is the right place to post this, but here it goes.
> I am a high-school math teacher from greece. I was working with my
> students on the limit:
> 
> Limit[ (Sqrt[7x^4+6x+5] - Sqrt[7x^4+3x+3]) * Sqrt[63x^2-5x+20, x->
> Infinity]
> 
> The result we got for that is 9/2. Mathematica 4.0 will produce the
> opposite result: -9/2.
> If anyone is interested on the way we calculated this limit, please
> e-mail me and I will be more than happy to send you a copy of our
> computation, since it would take me ages to type this in ASCII and
> furthermore it would be, at least, annoying to the viewer.
> 
> Excuse my rusty english, best regards
> Kostas Raptis
> 


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